题目

—Is this your ruler? —No,. It'sruler. A . this isn't; Li Lei's B . this is; Li Lei's C . it isn't; Li Lei's D . it is; Li Lei 答案：C9．已知O1，O2，O3分别是正方体ABCD-A1B1C1D1的三个面A1B1C1D1，CC1D1D，BCC1B1的中点，求异面直线AO1与O2O3所成角的大小．