A:∠1=∠2
B:∠D+∠ACD=180°
C:∠D=∠DCE
D:∠3=∠4
——How long have you ________ your penfriend?——For five days. I received it last Sunday.
人类的遗传病多种多样,一部分是致病基因引起的,当致病基因是____基因纯和时,个体表现为有病.