_______China’s first manned space flight took place?

A．When it was that

B．Was it when that

C．When was it that

D．Was it that when

下列高分子化合物老化后，属于物理性能发生变化的是（）。

A、塑料薄膜透光性下降

B、橡胶制品弹性消失

C、赛璐珞制品耐热性降低

D、胶鞋发硬