________ most wolves rely on their groups for food, shelter and companionship, they have developed a system of signals to keep the groups together.

1. A.
   Because
2. B.
   Although
3. C.
   Unless
4. D.
   Where

    most wolves rely on their groups for food ,shelter and companionship ,they have developed a system of signals to keep the groups together.

A．Because    B．Although   C．Unless      D．Where