单项选择题

A.M<SUB>1</SUB>+M<SUB>2</SUB>
B.M<SUB>1</SUB>-M<SUB>2</SUB>
C.M<SUB>1</SUB>·M<SUB>2</SUB>
D.M<SUB>1</SUB>/M<SUB>2</SUB>