改错 A cake made of wheat costs less than that made of rice .是

对的.是把that改为one
A cake made of wheat costs less than one made of rice.
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,