It is often said that the joy of travelling is______in arriving at your destination _____in the journey itself.

A．/ ; but

B．/ ; or

C．not; or

D．not; but

在两种温度不同的流体进行热量交换中，有一种流体是从气态被冷凝成为液态，温度变化并不大就称为冷凝器。