Which of the following is equal to Start Root 18 x superscript 7 baseline y superscript 6 baseline End Root?
a. 3 x superscript 5 baseline y superscript 4 baseline

b. 9 x superscript 6 baseline y superscript 5 baseline

c. 3 x superscript 5 baseline y superscript 4 baseline start root 2 end root

d. 3 x superscript 3 baseline y superscript 3 baseline start root 2 x end root