已知函数f（x）=x²e^ax，其中a≥0，e为自然对数的底数．

（Ⅰ）讨论函数f（x）的单调性；

（Ⅱ）求函数f（x）在区间[-1，0]上的最大值．

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Going on a trip to a far-away destination _________ be expensive and difficult, but it is very interesting.

A．can

B．must

C．should

D．will