Exponential problem!

[arocks]

a+b=5, a*b is not 0, is ax^4+bx^3=a+b?

1). ax^4=a
2). bx^3=b

[spoiler]IMO - C. But the OA is given as B. Not sure how we can solve it just by using B. Is the OA incorrect?[/spoiler]

[josephcho77]

(1) ax^4=a, x can be 1 or -1
when x is 1, ax^4 + ax^3 = a+b
but when x is -1, ax^4 + ax^3 = a-b

(2) bx^3=b, x can only be 1 thus ax^4+bx^3 is always a+b