Bronson owns a 24-unit apartment building. In 1997, he rented all of the units for $150 a month and had 100% occupancy. In 1998, he increased the rent by 10%, but he had a 10% vacancy factor. His annual effective gross income was approximately:

A: 10% more in 1997;
B: 10% more in 1998;
C: Equal in both years;
D: 1% more in 1997.

Answer: D: 1% more in 1997.