1. Given that in the acute triangle $\triangle A B C$, the distance from vertex $A$ to the orthocenter $H$ is equal to the radius of its circumcircle. Then the degree of $\angle A$ is ( ).
(A) $30^{\circ}$
(B) $45^{\circ}$
(C) $60^{\circ}$
(D) $75^{\circ}$
(2007, National Junior High School Mathematics League)
Answer: C. 

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