Based on the Following Figure

Multiple Choice

based on the following figure.
Each cell represents one combination of factor levels. The actual experiment assigns participants only to the combinations represented by the shaded cells. It is unknown whether the levels of the factors are fixed or randomly selected.
-Both A and B are random-effects factors. Each cell has 5 observations. The F_B ratio has degrees of freedom equal to?

A) 2, 3
B) 2, 10
C) 2, 24
D) 3, 72