2019-2020
1 8 s
4 s
( )
A a1a2 21
B v1v2 21
C a1a2 12
D x1x2 21
2
A
B
C
D
3
A3N4N5N
B2N4N6N
C1N3N5N
D4N6N8N
4 A A ( )
AAB
BAB
CA
DA
5 OB
B GAOBO
OAO OBO