Lauren has a paper with a seven-digit phone number that has the last three numbers smudged so that she cannot see them. The first four digits are 1, 7, 9, and 8, and the last three digits are the ones she cannot discern. How many possible phone numbers must Lauren dial if she is to be certain the she will eventually dial the correct number?

A. 1000
B. 100
C. 10
D. 30