各类营养物质在人体消化道内开始被消化的顺序是( )
①蛋白质 ②淀粉 ③脂肪 ④无机盐 ⑤纤维素.
A:①②③
B:①③⑤
C:③②①
D:②①③
已知数列{an},如果a1,a2-a1,a3-a2,⋯,an-an-1,⋯是首项为1公比为2的等比数列,那么an=( )
— Oh, dear! I forgot my best friend’s birthday.
— ________ You'd better keep it in mind next year.
分式x-1x+2的值为0时,x的值是( )