When Given Free Access to Playing with Blocks, Crayons, and Trucks

When given free access to playing with blocks, crayons, and trucks, Jeff plays with blocks about 50% of the time, plays with crayons about 30% of the time, and plays with trucks the remaining 20% of the time. According to the Premack principle, which of the following is true?

A) Playing with trucks will serve as a reinforcer for playing with crayons.
B) Playing with trucks will serve as a reinforcer for playing with blocks.
C) Playing with crayons will serve as a reinforcer for playing with blocks.
D) Playing with crayons will serve as a reinforcer for playing with trucks.