单项选择题
A.S
(L)
(L,S)
(L,(L))
(L,(L,S))
(L,(L,a))
(L,(S,a))
(L,(a,a)
(S,(a,a))
(a,(a,a)
B.S
(L)
(L,S)
(S,S)
(S,(L))
(a,(L))
(a,(L,S))
(a,(L,a))
(a,(S,a)
(a,(a,a))
C.S(
L)
(L,S)
(S,S)
(a,S)
(a,(L))
(a,(L,S))
(a,(S,S))
(a,(a,S))
(a,(a,a))
D.S
(L)
(L,S)
(S,S)
(a,S)
(a,(L))
(a,(L,S))
(a,(L,a))
(a,(S,a)
(a,(a,a))