求助，关于either的一题DS

sofarlee

100 student are taking both law and accounting. How many students are taking only accounting?
(1) 180 students are taking either law or accounting.
(2)50 students are taking law but not accounting.


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答案是c，但是解释说 "either law or accounting " includes people taking both subjects or only accounting or only law
eiher or不是二选一吗，怎么会包括两者都选，是不是题目解释错了？求nn解答