Solution-
CN is defined on boiling point criteria. Following are the boiling point range for C7, C8 and C9 (ref:Phase Behaviour of Petroleum Reservoir Fluids-Pederson)
from table 2.1
C7: 69.2-98.9 (degC)
C8: 98.8-126.1 (deg C)
C9: 126.1-151.3 (deg C)
and boling point for Benzene, Toulene, Ethyl Benzene, O/M/P-Xylene are as (ref: table 2.2- PEDERSON)
Benzene: 80.2 (deg C)
Toulene: 110.7 (deg C)
Ethyl Benzene: 136 (deg C) (source-Wikipedia)
m+p-Xylene: 138.8 (deg C)
o-Xylene: 144.5 (deg C)

So accordingly Benzene can be added to C7, Toulene to C8 and Ethyl Benzene, O/M/P-Xylene to C9.