A. The sun heats the air at the equator, followed by hot air rising and moisture condensing B. Air moves towards the poles as it cools
C. The dry air moves back to the equator, picking up a lot of moisture on the way
D. The air sinks around 30° latitude and is very dry.

B. Air moves towards the poles as it cools

Explanation:

The image represent convection cell or Hadley cell is a natural phenomenon in the atmosphere that occur due to difference in the density within a body of liquid or gas. These differences in density causes rise and fall in currents.

The process in step A refers to when hot air moves up and in the step B, the hot air encounters a cold surface, it direct exchange the heat with cold surface and the air cools down and move towards the poles.

Hence, the correct answer is "B. Air moves towards the poles as it cools".

littering streams           is the answer

there are so many examples for that in different areas, like biology experiment carried out in our lab recently.

here's one link: