Using activity$-$cost rates rather than department indirect$-$cost rates to allocate costs results in different product costs when $\_\_\_\_\_\_\_\_.$

Question $22$ options:

a$)$ a single activity accounts for a sizable portion of department costs

b$)$ there are several homogeneous cost pools

c$)$ different activities have the same cost$-$allocation base

d$)$ different products use different resources in the same proportion

answer a & d are incorrect according to my teacher