In the direct communication mode, the two communication primitives usually provided by the system are

A$$ send$($receiver$,$ message$),$ receive$($sender$,$ message$)$

B$$ send$($sender$,$ message$),$ receive$($receiver$,$ message$)$

C$$ send$($mailbox$,$ message$),$ receive$($mailbox$,$ message$)$

D$.$ send$($mailbox$,$ message$),$ receive$($receiver$,$ message$)$