本题考查英国的两党制。

Its decisive characteristic is that the main opposition party forms an alternative, or "shadow," government, ready at any time to take office, in recognition of which the leader of the opposition receives an official salary.

它的决定性特点是主要的反对政党形成另一个“隐蔽的”政府，其随时准备上台，当反对党领袖得到政府工资时，这个隐蔽的政府也就得到认可了。故选A。