itmustbemorning,thebirdsaresinginga.becauseb.asc.sinced.for请简述四者区别

Question

it must be morning,____ the birds are singing
a.because
b.as
c.since
d.for
请简述四者区别

Answer 1

肯定是早上,因为鸟儿在歌唱.
for表示推断性原因.
A必然原因,回答why
B次于because的原因,语气没那么重.
C显然的原因（显而易见的原因.Since you are a boy,you should do it.)
是早上,鸟儿歌唱并不是必然原因的.
The spring has come,for the birds are sining.