焚烧炉在燃烧时炉膛内的压力情况是（）。
A.正压
B.负压
C.大气压力
D.不一定
参考答案：B
The government is asked to improve school bus safety, ______? [ ]
A. didn't it
B. hasn't it
C. isn't it
D. doesn't it
RS-232C关于机械特性的要求，规定使用一个______根插针的标准连接器。
