The day breaks，______ the birds are singing.
A. because B. as C. since D. for
The day breaks，______ the birds are singing.
A. because B. as C. since D. for
D
若选A、B、C，强调The day breaks(天亮)的直接原因或者显而易见的原因就是the birds are singing(鸟叫)，显然这是不成立的，只有for引出的句子作为一种附加说明才成立。