Mary's score on the test is the highest in her class; she --

Mary's score on the test is the highest in her class; she must have studied very hard.
玛丽的评分测试是班上最高的,她必须学习非常努力.
Mary's score on the test is the highest in her class; she may have studied very hard.
玛丽的评分测试是班上最高的,她可能很难努力.
你自己看吧,还是must比较通顺一点.