For an animal to move in one direction, its muscles must do all of the following except

a. overcome the effects of friction.
b. exert a force in the opposite direction.
c. spend energy.
d. overcome the effects of gravity.
e. exert a force in the same direction.

answer: c

explanation:

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.

imagine a line passing through the center of earth that goes through both the north pole and the south pole. this imaginary line is called an axis. earth spins around its axis, just as a top spins around its spindle. this spinning movement is called earth's rotation. at the same time that the earth spins on its axis, it also orbits, or revolves around the sun. this movement is called revolution.