OG10-8

8. Lacking information about energy use, people tend to overestimate the amount of energy used by equipment. such as lights, that are visible and must be turned on and off and underestimate that used by unobtrusive equipment, such as water heaters.

(A)  equipment, such as lights, that are visible and must be turned on and off and underestimate that

(B)   equipment, such as lights, that are visible and
                must be turned on and off and underestimate it when

(C)  equipment, such as lights, that is visible and must be turned on and off and underestimate it when

(D)  visible equipment, such as lights, that must be
                turned on and off and underestimate that

(E)   visible equipment, such as lights, that must be
                turned on and off and underestimate it when

答案中：Moreover, D solves the agreement problem of A and B by omitting the to be verb used with visible and placing visible before equipment',是什么意思？