问题
选择题
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
答案
B
题目分析：句意：有必要详细地解释你的理论，否则观众就不会明白你想要表达的意思。A. on purpose故意地；B. at length详细地；C. at random随意；D. on average平均。故选B。
考点: 考查介词短语的用法。