A、The atmosphere surrounding the Earth. B、Water from oceans and lakes. C、Energy from the Sun. D、Greenhouse gases in the sky. C问题

We experience different forms of the Sun’s energy every day. We can see its light and feel its warmth. The Sun is the major source of energy for our planet. It causes the evaporation of water from the oceans and lakes. Sunlight also provides the energy used by green plants to make their own food. These green plants then provide food for all organisms on the Earth.
    Much of the energy that comes from the Sun never reaches the Earth’s surface. It is either reflected or absorbed by the gases in the upper atmosphere. Of the energy that reaches the lower atmosphere, 30% is reflected by clouds or the Earth’s surface. The remaining 70% warms the surface of the planet, causes water to evaporate, and provides energy for the water cycle and weather. Only a tiny part, approximately 0.023%, is actually used by green plants to produce food.
    Many gases found in the atmosphere actually reflect heat energy escaping from the Earth’s surface back to the Earth. These gases act like the glass of a greenhouse in that they allow energy from the Sun to enter but prevent energy from leaving. They are therefore called greenhouse gases.
    When sunlight strikes an object, some of the energy is absorbed and some is reflected. The amount reflected depends on the surface. For example, you’ve probably noticed how bright snow is when sunlight falls on it. Snow reflects most of the energy from the Sun, so it contributes to the low temperatures of winter. Dark-colored surfaces, such as dark soil or forest, absorb more energy and help warm the surrounding air.
Questions 19 to 21 are based on the passage you have just heard.
19. What is the root cause for weather changes on the Earth according to the passage?
20. Why does only a small part of the Sun’s energy reach the Earth’s surface?
21. What do we learn from the passage?