Landing issue..suggestions needed

Dear Friends,

My PR has been approved and i am planning to do the landing very soon, may be in December this year. I suddenly realised that i made an error during my application process...the problem goes like this..
In the background declaration part of the application, i didnt understand the question about previous visa denials..probably i read it wrong..and thought the CIC intended to know only about the Canadian visa denials...so i answered it as NO denials...but i realised that i erred in reading the question...and i didnt report my US student visa denials...although subsequently i got an US visa and have even visited the US..

My question to the learned members here is, what will be the impact of such a mistake from my side...and what are the chances of the immigration officer asking me about visa denials....and if i will be asked...what should i say..i know im in a spot of bother just because of missing a part of the question...

Suggestions are highly appreciated..

Thank you all