Blank is the amount of water vapor in the air and it can measured with a blank

The 5th term in a series of even number that begin 3x

What are the answers for the following questions?

Acivilization exists for several hundred yeaars, developing its own language. such a civilization probably has been: a. farming corn b. suffering from a lack of contact with the outside world. c. isolated by mountains or some other physical barriers. d. starving from lack of trade.