_____ ashes from the volcano in Iceland have caused problems for air traffic and business, scientists don't think it will have long-term effects on human health.

_____ ashes from the volcano in Iceland have caused problems for air traffic and business, scientists don't think it will have long-term effects on human health.

1. A.
   Since
2. B.
   As
3. C.
   While
4. D.
   Because

_____ ashes from the volcano in Iceland have caused problems for air traffic and business, scientists don't think it will have long-term effects on human health.

       A. Since                              B. As                           C. While                      D. Because

_____ ashes from the volcano in Iceland have caused problems for air traffic and business, scientists don't think it will have long-term effects on human health.