单项选择题

A.18
B.18π
C.<br /><img src="https://imgtest.ppkao.com/ExamPaperRes/2018/2/27/0ead6f93060547278a693829d9636679/ecae8647-20b9-432e-b6c5-c59f42ea774d.jpg">
D.<br /><img src="https://imgtest.ppkao.com/ExamPaperRes/2018/2/27/0ead6f93060547278a693829d9636679/5621854e-7f58-4f9d-90fd-375e7bbd3b3b.jpg">