介词填空。

l. That was an important date ________ which we set up our own business.

2. This was a difficult period ________ which we went through many sufferings.

3. He returned to the village ________ which he spent his happy childhood.

4. Do you know the reason ________ which he is absent?

5. Is this the car _______ which you paid a high price?

投影面平行面是指平行于一个投影面的平面，这种平面自然（）于另外两个投影面。

A、平行

B、正平

C、垂直

D、侧平