How many moles are there in 7 Liters of oxygen gas at STP?

Iknow the answer is a but i don't know why

Why is the vapor pressure of a warm lake higher than the vapor pressure of a cold lake? o a. warm water has a greater heat of vaporization. ob. warm water evaporates more quickly. cool water evaporates more quickly. od. cool water has a greater heat of vaporization.

Δu of , in kj/kg, as it isto k, (a)as a of , (b) at , (c) at .