I _______ this computer five years ago.
A:buy
B:bought
C:have bought
D:has bought
等腰三角形的两边长分别为1和2,则其周长为( )
与叶肉细胞相比,细菌细胞缺少的结构是:( )
①细胞壁 ②细胞膜 ③细胞质 ④成形的细胞核 ⑤叶绿体.