You should tell the truth. The officer will ask the following questions:
1. where do you live?(the first question, must ask)
2. why do you come to US(second question, must ask)
3. what company? what position?
4. do you have the qulification?
5. when will you come back?
6. when did you get Canadian citizenship?
7. when and how did you enter Canada?
you might be asked to show:
1. the interview invitation letter(email print is ok)
2. the degree certificate(you can say you did not bring it with you)
As long as you are telling the truth, there is no problem at all. The officer's questions are to detect whether you are an illegal immigrant using a fake Canadian passport. Therefore, if you try to cover up something, you will be in trouble.