某塑料管无毒、价廉，但抗冲击强度较差。经改性后具有较好的抗冲击性能和具有耐温度变化性能、抗蠕变性能，采用熔接方式连接，此管材为（）。

A．硬聚氯乙烯管

B．聚乙烯管

C．聚丙烯管

D．聚丁烯管

We began to open to the outside world in the ate 1970s.        ,  there have been great changes in the country.

A．As a result of

B．As a result

C．First of a11

D．After all