2. You have a great singing voice. You seem relaxed on stage. The most effective way to combine these sentences would be:
a. You have a great singing voice, and you seem relaxed on stage.
b. You have a great singing voice, but you seem relaxed on stage.
c. You have a great singing voice, so you seem relaxed on stage.

I would say it is a.

Explanation:

B wouldn't make sense, since both parts of the sentence are positive, so no need for a contradiction.

C wouldn't make that much sense, since having a great singing voice might not be the main reason why you seem relaxed.

A would make the most sense since it is stating 2 distinguishable features about you.