which of the following expressions is equal to 1?
a. 5^0 x 6^-3 x 6^3 x 5^1
b. 5^2 x 6^-2 x 6^3 x 5^-3
c. 5^2 x 5^-2 x 6^0 x 6^2
d. 5^-1 x 6^5 x 6^-5 x 5