High-speed trains travel at 300 kilometers an hour，which is about normal trains．
A.four times the speed asB.the speed four times ofC.four times as the speed ofD.four times the speed of在线课程D
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。