People differ physically from one another because:
their alleles differ
their environments differ
their languages differ
their images differ

"a government in which power is divided between state and national levels". federalism is a system of government in which entities such and state and province, share power with the national government.

d) a week

because the land run of 1889, although not without precedent in the history of the west, further, their men were aboard the first trains, ready to jump off and begin staking their claims. oklahoma city, edmond, and guthrie were all so affected.

you didn't list the options. however, a few communist countries today are: the people's republic of china, north korea, and cuba.