—Not yet. Tom _____it.
A:wrote
B:is writing
C:writes
D:will write
建立下列复杂反射的正确顺序是( )
(1)铃声响,狗就分泌唾液
(2)吃肉时,狗分泌唾液
(3)铃声成为条件刺激
(4)吃肉时,铃声响,狗分泌唾液
(5)多次反复.