Hallo HongHu, think that B) by itself is INSUFF.
From B) we know that 1/8 attended morning session, but we do not know how many people have atttended BOTH sessions.The stem says AT LEAST ONE OF it doesn't clarify if they have attended only one session
If they have attended both then they would already been included in the people who have attended the afternoon session. See this:
People who attended any
= people in morning only + people in afternoon only + people in both
= people in morning only + people who have attended the afternoon session no matter if they attended both
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