若双曲线 $\frac{x^2}{9}-\frac{y^2}{4}=1$的两条渐近线恰好是抛物线y=ax²+$\frac{1}{3}$的两条切线，则a的值为（　　）

    One afternoon, I went into an art museum. While I was ____ for my friend, I saw a young couple (夫妻). The woman was ____ the paintings. The man was quiet all the time. I .thought the man was very nice because nobody would like to be bothered (被打扰) ____ enjoying the paintings.

    I met them a few ____ as I moved through the different rooms of the art museum. Each time I heard the woman talking, I moved away ____

    I was paying for some ____ at the museum shop when the couple walked slowly to the exit (出口). Before they left, the man ____ out a cane (拐杖). "He is a great man," the worker of the shop said. "He ____ not to change his life after his eyes got hurt. He can't see ____ now. He and his wife-come here whenever there is a new art show. "

    "But what can he get from the art? He can't see!" I said.

    "You are wrong. He sees a lot, more than you and I do," the worker said. "His wife ____ each painting so he can see it."

果核透明、味道甜美的“冰糖心苹果”产于阿克苏地区（如图），结合图文材料，“冰糖心苹果”味道甜美的主要原因是（   ）