— Look at the boy running on the ground, Is it David?

— It _____ be him, he has just gone to the teacher's office. [ ]

A. must

B. can't

C. could

D. may

下列关于安全边际及安全边际率的说法中，正确的有（）。

A.安全边际是正常销售额超过保本销售额的部分

B.安全边际率是安全边际与正常销售额的比

C.安全边际率和保本作业率之和为1

D.安全边际率数值越大，企业发生亏损的可能性越大