One Day,a Classmate with Whom You've Been Trying to Go

One day,a classmate with whom you've been trying to go out socially gives you his/her number on a napkin (it was the only thing you had handy at the time) .Unfortunately,when you wash your jeans,you forget to take out the napkin,and now all you can piece together of the phone number are the first six digits.If you go to the phone and dial the first six digits plus "0",then dial the first six digits plus "1",then dial the first six digits plus "2",etc.until you dial the correct number,you are solving the problem by

A) using heuristics.
B) rote.
C) trial and error.
D) insight.