A recent national study of the public schools shows that there are now one microcomputer for every thirty-two pupils, four times as many than there were four years ago.
我觉得C中应该用there were, 因为前面说得是每....人有一台电脑,虽然是表达为there is one....,但其隐含的意思是有很多台电脑,而不是只有一台,而且,as...as 后面比较的也是整体情况,如果要用there was, 后面就应该将one for every... 补充完整,否则只能用there were.