B
长句中间的部分，“as most classmates had expected”是插入的修饰语，可以不看，句首的长句整体构成最后一句“made her parents very happy.
”的主语。
实际上就是“Mary being chosen as the best student in her college made her parents very happy.”
A的话，单独构成了一句话，是不能做整个句子的主语的。
选A is being 有腔调的意思
im just being real 我只是实话实说而已
选B
A
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