The Grammar Whose Productions Are

The grammar whose productions are
? if id then
? if id then else
? id := id
Is ambiguous because
A- the sentence if a then if b then c:= d has two parse trees
B- the left most and right most derivations of the sentence if a then if b then c:= d give rise to different parse trees
C- the sentence if a then if b then c:= d else c:= f has more than two parse trees
D- the sentence if a then if b then c:= d else c:= f has two parse trees

A) a
B) b
C) c
D) d