To do some voluntary work for the 2nd Youth Olympic Games is ______ excellent chance for us to serve our hometown—Nanjing.

A．a

B．an

C．the

D．不填

葛根改善脑循环作用的主要成分是

A．葛根总黄酮

B．生物碱

C．挥发油

D．氨基酸

E．多糖