问题：

[单选] Which option is true when calculating round-trip delay in IP SLA operations？（）

A . The processing time on the end routers is only assessed for operations that involve the responder.
B . The processing time on the end routers is only assessed for operations that involve the transmitter.
C . The processing time on the end routers is only assessed for operations that involve both the responder and the transmitter.
D . The processing time on the end routers is not assessed for neither the responder nor the transmitter.