DIRECTIONS: In the letter series given below, some letters are missing. The missing letters are given in the proper order in one of the responses a,b,c and d. Find our the correct response(choice):
a - a - - c a b a - - c - b a - c
1. a a c b c a b
2. b b c b c a b
3. b b c c b a b
4. a a c c b a b