What does this sentence mean?

1. What does this sentence mean?

The one thing I can do is to tell the truth.

A.The only thing I can do is to tell the truth.

B. One of the things I can do is to tell the truth.

2.Are these sentences natural?

A. One star after another was covered by the cloud.

B. One after another the stars in the west were covered by the cloud.

C. He raised his hands one after the other.