—Mr. Smith has got a very good job in the famous company.

—_____ he looks so happy.[ ]

A. It's certain  

B. That's because

C. No doubt  

D. No wonder

已知函数f(x)＝a^x＋x²－xlna(a>0，a≠1)．

(1)当a>1时，求证：函数f(x)在(0，＋∞)上单调递增；

(2)若函数y＝|f(x)－t|－1有三个零点，求t的值；

(3)若存在x₁、x₂∈[－1，1]，使得|f(x₁)－f(x₂)|≥e－1，试求a的取值范围．