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
Don average
网友回答
A. on purpose
(答案→)B. at length
C. at random
D. on average
解析：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。