(单选题)16: As long as the sun shines, the earth will not run out of energy. The sun pours more energy on earth than we can ever use. Most of that energy comes to us as heat and light. Energy from the sun is called solar energy.    Anything to do with the sun is called "solar". The word began with the Roman word for the sun and their god of the sun, who was called Sol. Solar energy is a safe kind of energy. It doesn't make pollution or have dangerous leftovers. That is why scientists and investors are experimenting with ways of harnessing（治理利用）the sun to do some of the jobs fossil fuels have been doing.    But to make the sun do work like that, they have to solve some problems. They have to collect the sun's energy. Collecting sunshine is not easy, unless you are a plant. Sunshine is not easy to store, either. You can't fill a tank with it or put it in the wood box. You can not move it through a pipe or a wire. You can not just turn it on. (4)、What are the problems facing scientists who want to use solar energy?

A: It is not easy to collect.

B: It is not easy to store.

C: It is not easy to find out.

D: both A and B.