She did very well on the exam. She <b>must</b> have studied hard. (1)<br/><br/>=She did very well on the exam. She <b>should</b> have studied hard. (2)<br/><br/>=She did very well on the exam. She <b>ought to</b> have studied hard. (3)<br/><br/>Is that right? If not, what is the difference btw them?<br/><br/>Quoc