In a Memory Study Using a 2x2 Factorial, One of the Factors

In a memory study using a 2x2 factorial, one of the factors is the presentation rate of the words, the two levels being 2 and 4 seconds per item. The researcher finds that recall is 98% accurate at 2 seconds per item and 99% accurate at 4 seconds per item (not a statistically significant difference) . The most reasonable conclusion is that

A) a ceiling effect has occurred
B) an interaction has occurred
C) presentation rate has no significant effect on memory
D) there is a main effect for presentation rate