B is the correct answer
sqrt (x^8 y^18)
(x^8 y^18) ^(1/2)
using the power of powers we multiply the exponents
x^ (8*1/2) y^ (18*1/2)
x^4 y^9
we have to be careful here because if y<0 this does not hold true
y^18 is positive y^9 can be negative
so we have to take the absolute value of y
we don't have to do this for x since x^4 is positive and x^8 is positive
x^4 |y|^9
Choice A
