If Z is a compressibility factor, van der Waals equation at low pressure can be written as:

Question

( A ) $Z = 1 - \frac{Pb}{RT}$
( B ) $Z = 1 + \frac{Pb}{RT}$
( C ) $Z = 1 + \frac{RT}{Pb}$
( D ) $Z = 1 - \frac{a}{VRT}$