The russians halted their offensive near warsaw and let the germans massacre the home army, while

The Soviet Union not only refused to condemn the German action but also launched its own invasion against Poland 16 days after the Nazi attack. Similarly, Germany was silent about the attack launched against Finland by the Soviet Union, during the so-called Winter War, in December 1939. Under this pact in June 1940 the Soviet Union also annexed Estonia, Lithuania and Latvia , immediately after the German triumph in the battle of France, and simultaneously annexed territories of the Kingdom of Romania.

Explanation:

a big wall across the mexican border so no drugs, terrorist, or illegal immigrants can come through : )    

just finished this it is b. centrifugal force