1.It is because he is too young ____he does not understand i

1.It is because he is too young ____he does not understand it.
A.WHEN B.therefore C.that D.then
2.He had me ____five letters for him.
A.to type B.typing C.type D.typed
3.Did they sleep ____all the windows closed?
A.when B.that C.for D.with
5.____finished the job,they worked more hours.
A.having been not B.Not having C.Having not D.Not having been