Opel is the main European arm of which US automaker?
No. Opel is the European arm of the General Motors corporation. While several Opel models are unique to their brand, they also badge engineer several foreign vehicles (manufactured by GM subsidiaries or by companies with which GM has a cooperative agreement with). For example, the second generation (1995+) Chevrolet Blazer was also manufactured by Opel (albeit in Indonesia, rather than Europe).
The arm is made up of two main muscles (biceps & triceps) for the arm to move one of these muscles must contract and the other must relax. Which muscle does which depends on which way you are moving your arm. Additional answer This does not mention the muscles in the forearm, which mainly move the hand, or the muscle in the shoulder, deltoid, which lifts the arm.