如果（m+3）x2-mx+1=0是一元二次方程，则m______．

浸泡中药饮片的水温不宜超过（）

A.20℃

B.30℃

C.40℃

D.50℃

E.60℃

下列可制食品袋、药物包装材料、日常用品、管道、绝缘材料等的塑料是（　　）

A．聚乙烯

B．聚氯乙烯

C．聚苯乙烯

D．聚丙烯

按要求完成句子用现在完成时改写下列句子，使A,B两句意思基本一致。每空限填一词。

1. A. We came to our school in 1980.

B. We ________ ________ ________ our school since 1980.

2.A. The old woman died four years ago.

B. The old woman ________ ________ ________ for four years.

3. A. The meeting began two hours ago.

B. The meeting ________ ________ ________ for two hours.

4. A. I borrowed two books last week.

B. I ________ the two books for a week.

5. A. When did the bus arrive here?

B. How long ________ the bus ________ here?

铲齿成形铣刀的刀齿廓形

当其他因素不变，目标利润因素单独变动时，会使保本点指标发生相应变动，但不改变保利点。