边长为1的正六边形的外接圆半径是____ .
I will tell him to call you as soon as he back.
在孟德尔的豌豆杂交实验中,必须对母本采取的措施是( )①开花前人工去雄 ②开花后人工去雄 ③自花受粉前人工去雄 ④去雄后自然受粉 ⑤去雄后人工授粉 ⑥授粉后套袋隔离 ⑦授粉后自然发育