烧穿的主要原因是（）等。A、焊接电流过大B、焊接速度过小C、坡口间隙过大D、焊接速度

单价的计算方法主要根据( )事先约定的工程价格的计价方式确定。

A．施工方和承包人

B．分包人和承包人

C．发包人和分包人

D．发包人和承包人

The speaker talked about sports in general and about football ________.[ ]

A. in a word  

B. in particular

C. in return 

D. in demand

The twins were so _______ in their TV play that neither of them heard the door ring.

A. adapted        B occupied      C absorbed           D concentrated

面向对象的模型中，最基本的概念是对象和【 】。

简述凝汽器灌水找漏灌水前准备工作。