Can't you discount B for the same reason given for A and C

The explanation says we can ignore A because we don't know how the difference in advanced degrees makes a difference in care between hospitals or which hospitals in the study have the higher concentration of advanced degrees. The explanation says we can ignore C because we don't know how experience changes care from one hospital to another and which hospitals have more experienced staff. This logic seems to apply to B. Without knowing which hospitals are stricken by more sever illnesses, how can we apply this to studies?