下列案例的输出结果是什么？（ ）
t1=(1,2,3,4,5,6,7) t2=("a","b","c","d","e","f")
a1=t1[2:]
a2=t2[2:5]
s=a1+a2
print(s)
(3, 4, 5, 6, 7, 'c', 'd', 'e')
( 4, 5, 6, 7, 'b','c', 'd', 'e')
(1,2,3, 4, 5, 'c', 'd', 'e')
(3, 4, 5, 6, 7, 'a', b', 'c')
