请用不超过150字的篇幅，概括出给定资料所反映的主要问题。
要求：概括全面，层次分明，条理清楚，表述简洁。

One night 18 percent of the female officers on a police force were on duty. If 180 officers were on duty that night and half of these were female officers, how many female officers were on the police force ？（）

A.90

B.180

C.270

D.500

E.1，000