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.
A. that B. what C. which D. how

【正确答案】 B

【答案解析】[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。