司钻法压井第一循环周中，压井排量保持不变，调节节流阀使（）等于初始循环总压力并在整个循环周保持不变。

A、立管压力

B、套管压力

C、圈闭压力

D、环空压力

A certain truck manufacturer must have sales of $4.5 billion to break even this year. If the manufacturer had sales of $ 1.9 billion for the first half of the year, then, to break even for the year, its sales for the second half must be approximately what percent higher than its sales for the first half ？（）

A．11%

B．16%

C．27%

D．37%

E．58%