What is the missing number in the series: 2, 4, 8, 16, ?, 64?

A. 32

Clock is to Time as Thermometer is to ?

A. TEMPERATURE

A, C, E, G, ?

A. I

FISH is to SWIM as BIRD is to ?

A. FLY

Identify the missing letter in the series: a, e, i, o, ?

A. U