丙烯干燥器备用速度过快，波动较大的是（）。

A．碳三加氢反应器

B．高压脱丙烷塔回流罐液位

C．低压脱丙烷塔

D．碳二加氢反应器

The first time I remember noticing the crossing guard was when he waved to me as I drove my son to school. He (31) me with a puzzle-all because he waved to me like someone does on seeing a close friend. A big, (32) smile accompanied his wave. For the next few days I tried tO study his face to see if I knew him. I didn’t. Perhaps he had (33) me for someone else. By the time I contented myself with the conclusion that he and I were strangers, we were greeting each other warmly every morning like old friends.

Then one day the (34) was solved. As I approached the school he was standing in the middle of the road (35) his stop sign. I was in live behind four cars. Once the kids had reached the safety of the sidewalk, he lowered his sign and let the cars (36) . To the first he waved and smiled in just the same way he had done to me over the last few days. The kids already had the window down and were happily waving their reply. The second car got the same (37) from the crossing guard, and the driver, a stiff-looking businessman, gave a brief, almost awkward wave back. Each following car of kids on their way to school (38) more heartily.

Every morning I continued to watch the man with interest. So far I haven’t seen anyone (39) to wave back. I find it interesting that one person can make such a difference to so many people’s lives by doing one simple thing like waving and smiling warmly. His (40) armed the start of my day. With a friendly wave and smiling fade he had changed the feelings of the whole neighbourhood.

40（）。

A. effectiveness

B. cheerfulness

C. carefulness

D. seriousness