A two-way ANOVA is conducted to compare detection accuracy based on target size (small or large) and observer age (old or young). Which of these is a null hypothesis for a main effect of this ANOVA? a. H subscript 0 colon space mu subscript s m a l l end subscript not equal to mu subscript l a r g e end subscript b. H subscript 0 colon space mu subscript s m a l l end subscript equals mu subscript l a r g e end subscript c. H subscript 0 colon space t h e r e space i s space n o space i n t e r a c t i o n d. H subscript 1 colon space mu subscript o l d end subscript equals mu subscript y o u n g end subscript