B.a
句意:据说,那条河上又要修建一座桥.
a/an +序数词+名单.表示"又一个,再一个"
the +序数词+名单.表示顺序"第几个..."
如:You will need a second pair of shoes.
你需要再准备一双鞋.
来自《现代英汉综合大词典》
The first day of a week is Sunday.