Đề ôn thi Đề thi vào lớp 10 Trường Phổ thông Năng khiếu môn Tiếng Anh - Mã đề PTNK-171

The Sun, at the center of our solar system, provides the heat and light essential for life on Earth. But what is this massive ball of fire made of? It isn't solid or liquid, but an incredibly hot ball of gas. The Sun is (21) __________ giant star, and like others, its composition is simple yet fascinating. Understanding its components helps us comprehend the processes that power our solar system.

Scientists have studied the Sun's light to learn its ingredients. It is mainly (22) __________ of two primary gases: hydrogen and helium. Approximately 74% of the Sun’s mass is hydrogen, and about 24% is helium. These two elements together make up 98% of the Sun. The small remaining percentage consists of a mix of heavier elements like oxygen and carbon.

This transformation happens deep inside the Sun’s core. Here, the temperature and pressure are immense; as a result, hydrogen atoms are squeezed so forcefully that they fuse into helium. This process, nuclear fusion, (23) __________ an enormous amount of energy as light and heat, which then travels through space to our planet.

Early astronomers explained that the Sun also contained trace amounts of other elements too. This discovery was crucial for understanding the life cycle of stars. However, compared to the vast amounts of hydrogen and helium, these heavier elements are just a drop in the ocean, representing less than 2% of its mass.

The conditions in the Sun's core are essential. If the Sun’s core were not so hot and dense, this fusion reaction (24) __________. The balance between gravity pulling inward and the energy from fusion pushing outward is what keeps our Sun stable, allowing it to shine for billions of years.

It’s a question many young people might ask: why go to the dentist when nothing hurts? If you're not experiencing pain, sensitivity, or a nagging toothache, a trip might seem unnecessary. Isn't the dentist for fixing problems? [I] Like maintaining a car before it breaks down, proactive dental visits are essential for a healthy smile long-term.

The truth is, many dental issues begin subtly, without immediate pain signals. A small cavity can develop slowly over months or years, gradually eroding enamel. By the time you feel pain, decay has often reached sensitive inner layers, making the problem more severe and complex, potentially requiring a root canal instead of a simple filling. Similarly, gum disease, affecting tissues supporting your teeth, can progress unnoticed, leading to bleeding gums and eventual tooth loss if not addressed early. [II]

This is where preventive dentistry shines. Regular check-ups, typically every six months, allow your dentist to spot silent problems before they escalate. During visits, dentists don't just look for cavities; they also check for gum disease, oral cancer, and other issues. Professional cleanings remove plaque and tartar buildup even the best brushing can miss, preventing cavities and inflammation. For teenagers, these habits are vital because your permanent teeth must last a lifetime. Caring for them now saves significant pain and costly treatments later. [III]

In many cultures, including Vietnam, the tendency has been to visit a dentist only when pain becomes unbearable. This reactive approach often means problems are already advanced, requiring more intensive and expensive interventions. [IV] However, a growing movement seeks to shift this mindset. Dental chains like Kim Dental are actively educating the public about the benefits of routine, preventive check-ups. They emphasize that investing time in regular care can prevent much larger issues, leading to better oral health for the entire community.

So, while it might seem odd to visit a dentist when you feel perfectly fine, it’s one of the smartest health decisions you can make. Embracing preventive dental care now means investing in your future self. It’s about ensuring your teeth remain strong, healthy, and pain-free, allowing you to enjoy food, speak clearly, and smile confidently for decades. Don't wait for pain to be your signal; make proactive dental care a part of your routine and protect your lifelong smile.