问题补充：
High-speed trains travel at 300 kilometers an hour，which is about normal trains．
A. four times the speed as
B. the speed four times of
C. four times as the speed of
D. four times the speed of
答案：
A. four times the speed as
B. the speed four times of
C. four times as the speed of
(答案→)D. four times the speed of
解析：考查倍数的表达方式。 倍数表达方式有 A is three times as big as B / A is three times the size of B/ A is twice bigger than B/ A is three times what B is. 参照句式可知D正确。
