4. Pure gaseous reactant A (CAO = 100 millimol/litre) is fed at steady rate into a mixed reactor where it dimerizes (2A ? R). For a certain gas feed rate the concentration of A in the exit stream is found to be 50 millimol/litre. The fractional conversion of A is
A. 66.67%
B. 40%
C. 33.33%
D. 50%