Earth's tectonic plates
A.
do not move.
B.
move only up and down.
C.
are constantly in motion.
D.
move only once.

The answer is C

Explanation:

Tectonic plates, the massive slabs of Earth's lithosphere that help define our continents and ocean, are constantly on the move.

try using substitution:

x + y - 6 = x - y. (i'm not giving you the exact answer). since 0 = x - y, then sub 0 in the equation with x - y. combine your like terms, and then use inverse operations, and you'll have your answer.

: -)