[误] A cake made of wheat costs less than that made of rice.

[正] A cake made of wheat costs less than one made of rice.

注：that 是从 the 变来的，属于定冠词的范围之内，所以它只能代表有定冠词的单数名词，如 The cost of oil is less than that of gas. 但凡是有不定冠词的单数名词，就不能用 that 代表，只能用 one 代表，如上举的两个例句：one=a cake，而 that=the cost。对于有定冠词的单数名词，也并不一定非用 that 代表不可，如 The step you have taken is that of much risk. 一句中的 that 就错了，应改为 one 才对，因为在动词 is 后应接 a step，所以只能用 one 代表。