what does M， Tr， R mean respectively in thread marks？

A.metric thread， trapezodial thread， sealing pipe thread

B.trapezoidal thread， sealing pipe thread，metric thread

C.trapezoidal thread， ，metric thread，sealing pipe thread

D.metric thread， trapezoidal thread， sealing pipe thread

正确答案：metric thread， trapezoidal thread， sealing pipe thread