It is necessary to explain your theory _____, otherwise the  audience won't make sense of what you are trying to get across.

A．on purpose

B．at length

C．at random

D． on average

实验室制下列各组气体，所用气体发生装置相同的是

A．H₂和CO₂

B．H₂和O₂

C．Cl₂和CO₂

D．Cl₂和H₂