A：Written, were
B：Written, there was
C：Writing, there was
D：Writing, were
我国古代四大发明之一的“黑火药”主要是由硝酸钾、硫磺、木炭组成，它属于（ ）
被誉为植物界的“大熊猫”的植物是（ ）
胚盘是鸟卵发育的起始部位．
