Single Letter Crossword

Together, going left to right, up and down, the letters spell a subject for clues 3, 8, and 10

1. Plural Ending
2. Universal Donor (Blood Group)
3. Caribbean _______ (sounds like)
4. Me, myself,
5. First dictionary entry
6. Looks like carpenter's square
7. S.O.___
8. Large party in Boston
9. Opposite of clue 4
10. Important day in WW2
11. Apple prefix
12. Second note in a c chord
13. First letter of clue 3

Answers: 1. S, 2. O, 3. C, 4. I, 5. A, 6. L, 7. S, 8. T, 9. U, 10. D, 11. I, 12. E, 13. S