A sentence with an independent clause and dependent clause(again)

Explanation:

The rain fell heavily when the clouds thickened and darkened.

That's an example of one.

The independent clause is The rain fell heavily. Those 4 words will stand by themselves as a complete thought. The dependent clause is when the clouds thickened and darkened.  

The dependent clause tells you when it rains heavily. It will not stand on its own.

i think it is b. hope this !