单选题
While the total number of farmers engaged in agricultural production is barely half ______ it used to be in 1959, the size of the average farm has tripled.
【正确答案】
B
【答案解析】half what it used to be相当于half the number that it used to be, what既代替先行词the number又引导一个从句，其他几个选择项均没有这个功能。