[正] A cake made of wheat costs less than one made of rice.
注:that 是從 the 變來的,屬于定冠詞的范圍之內,所以它只能代表有定冠詞的單數名詞,如 The cost of oil is less than that of gas. 但凡是有不定冠詞的單數名詞,就不能用 that 代表,只能用 one 代表,如上舉的兩個例句:one=a cake,而 that=the cost。對于有定冠詞的單數名詞,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 就錯了,應改為 one 才對,因為在動詞 is 后應接 a step,所以只能用 one 代表。