人体内二氧化碳浓度最大的地方是肺泡处的毛细血管。[ ]
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His father was pleased ___ his mark in the final exam.
A．at
B．with
C．to
D．for
已知集合，A=x|x≤1，B=x|x≥a，则A∪B=R，实数a的取值范围是（）．
