Not until _________ did man accept that fact that all living things must die.

A．did many years pass

B．many years past

C．many years had passed

D．many years have passed

某新建住房内铺设的自来水管用镀锌钢管50 kg，若保持管壁厚度、直径不变，改用密度为钢管密度的工程塑料制作，可使自来水管质量减少     kg；为了保证水管强度，在水管内径不变的条件下，应使横截面积为镀锌钢管的2倍，则需此塑料管     kg。