A:2
B:7
C:19
D:13−63
Jim tried his best to raise his spirits though he failed the exam.
人的肠道中也有一些细菌能够制造维生素B12和维生素K,它们和人是共生生活。
下图a→d表示连续分裂的两个细胞周期。下列叙述错误的是( )