Fusion is harder to control than fission because A. hydrogen atoms are lighter than uranium atoms
B. U-235 is more common than hydrogen
C. fusion reactions require extremely high temperatures
D. hydrogen atoms are harder to break apart than U-235 atoms

C

Explanation:

fission requires slowing down neutrons so they hit more nuclei (easy), but fusion requires speeding up nuclei so they hit each other (hard).

the old price per ounce is c/4.

the new price per ounce is c/3.6.

the ratio of the old price to the new one is

(c/4)/(c/3.6) = (c/c)×(3.6/4) = 0.9 . . = 9/10 . . "nine tenths" in either form

answer( sicar );