—Who do you think has made my room so dirty, Mom?—It  _____

—Who do you think has made my room so dirty, Mom?
—It  ______ be your younger brother.
A、must
B、shall
C、will
D、would
【正确答案】：A
【题目解析】：must 表示肯定的推测，句意：肯定是你弟弟弄脏的；因此A 符合题意。