答案为A
Owing to the unforeseen difficulties on the part of the suppliers ,we...
owing to= because of =on account of 介词短语,一般做状语,后面跟短语的原因
如：He came to school late owing to the traffice jam.
as由于,一般是指客观原因
如：As it is raining outside,we might as well have indoor sports.
because因为,比较直接的原因
如：He didn't come to school because he was ill.
since= now that 既然 表示大家都知道的原因
如：Since everyone is here,let's have the meeting.