[求助]Prep test2的NO.39

39.   (26492-!-item-!-188;#058&003168)

As it is with traditional pharmacies, on-line drugstores rely on prescriptions to be successful, since it is primarily prescriptions that attract the customers, who then also buy other health-related items.

(A) As it is with traditional pharmacies, on-line drugstores rely on prescriptions to be successful

(B) As with the case of traditional pharmacies, on-line drugstores rely on prescriptions to have success

(C) As is the case with traditional pharmacies, prescriptions are the cornerstone of a successful on-line drugstore

(D) As traditional pharmacies, so on-line drugstores rely on prescriptions to be successful

(E) Like traditional pharmacies, the cornerstone of a successful on-line drugstore is prescriptions

答案C，请问As is the case with traditional pharmacies在这里是什么结构，起什么作用，怎么解释啊？