门式钢管手脚架的顶层门架上部、连墙体设置层、防护棚设置处均必须设置水平架。
参考答案：对
He found it hard ______ his class.[ ]
A. to catch up with
B. to catch up
C. catch up with
丁肇中院士发现了第（）种夸克。
A.二
B.三
C.四
D.五
