PLEASE HELP ME WITH FACTORING!!!?

I have been trying to figure out these four questions all day and cant make sence of them. Please help me!?

1)) a^4 + 5a^3 - 50a^2

A: (a - 5)(a + 5)(a - 8)(a + 8)

B: (a - 5)(a + 5)(a^2 + 64)

C: (a^2 + 25)(a - 8)(a + 8)

D: (a^2 + 25)(a^2 + 64)

E: Nonfactorable

2)) -4c^2 + 12c + 40

A: (c - 2)(-4c + 20)

B: -4(c + 2)(c - 5)

C: -4(c + 2)(c + 5)

D: (-4c + 5)(c + 8)

E: Nonfactorable over the integers

3)) 5x^4 - 625x

A: 5x(x - 5)(x^2 - 5x + 25)

B: 5x(x + 5)(x^2 + 5x + 25)

C: 5x(x + 5)(x^2 - 5x + 25)

D: 5x(x - 5)(x^2 + 5x +25)

E: Nonfactorable

4)) x^3 - 5x^2 - 16x + 80

A: (x + 4)^2 (x - 5)

B: (x + 5)(x - 5)(x - 4)

C: (x + 4)(x - 4)(x - 5)

D: (x - 5)^2 (x + 4)

E: Nonfactorable