一道prep题不懂？请求解答

    Outbreak of Rift Valley fever occur irregularly in East Africa, several years apart. When outbreak do occur, they kill thousands of cattle. A livestock vaccine against the disease exists, but it's rarely used. It's too expensive for farmers to use routinely and since it's not affective until a month after vaccination, administering it after an outbreak begins helps very little. Neverthless, experts predict that use of vaccine will increase significantly within next few years.

      (A)Rift Valley fever is spread by mosquitos, but each outbrak is so widespread that it is impractical to control by using insecticides.

      (B)When an outbeak of Rift Valley fever occurs, unaffected countries often refuse livestock for the countries affected by the outbrreak.

      (C)It could take less than a month for producers of the vaccine to adjust their production operations to cope with a large increase in demand.

      (D)Many cattle farmers in East Africa are nomadic or live remote village, and such farmers, who have little access to modern vetering madicine, are partically hard hit by outbreak of Rift Vally fever.

      (E)Recent published research has shown that certain identificable climate condition are almost invaribly followed, within two to five months, by an outbreak of Rift Vally fever.

呵呵，问题忘粘过来了吧。是解释吧。

前提：vaccine is rarely used--原因：1.too expensive to use routinely  2.not affective until a month after vaccination

待解释现象：the use of vaccine will increase

那么一定是两个原因至少有一个被消除了。E说，out break of RV fever不可避免地要在一个已经认定的气候条件后两个月到五个月之内到来，所以农民已经确切知道了fever到来的时间，不用use routinely；且现在用疫苗的话，一个月后起效，刚好赶得上fever的到来。