A
B
B C
C D
D
B
B
4.
B2B. ( ) v
MN
MN 60 t1 t2
12 ( )
A 1 211 B 1 221
Ct1t211 Dt1t221
T12
qB T2
qB 2
T
12
12AB t1260
360T1t2260
360T2,t1t221,D ,C
D
5. abc
a ( )
A a N B b N
C c S D a N
N
S a a N A
D b bN
B c c N S C