请教一道数学题

zasdzccUID: 173579

P(x)=ax^4+x^3-b^2-4x+c.If P(x) increases with bound as x increases without
bound,then as x decreases without bound,P(x)
(A) increases without bound
(B) decreases without bound
(C) approaches zero from above the x-axis
(D) approaches zero from below the x-axis
(E) cannot be determined.
Answer:A
为何我对P(x)求出2次导数之后得到的答案为B?
Thanks.