A:Having told
B:Having been told
C:She had been told
D:Though he had been told
与植物细胞相比,草履虫缺少的结构是( )
①细胞膜 ②细胞壁 ③叶绿体 ④细胞质 ⑤细胞核 ⑥液泡.
下列各项中,能看做一个生态系统的是( )