—Is Bob still performing?

—I'm afraid not.He is said ________ the stage already as he has become an official.

A．to have left

B．to leave

C．to have been left

D．to be left

当换用不同技术特性的仪器时，平行观测期限可视换用仪器的（）变化情况而定，但至少不得少于（）。