A

    It's a beautiful day in summer. There are many people on the beach. Mrs Guo and her two sons are on the beach, too. Look! Mrs Guo is swimming in the sea. She can swim fast. What are her sons doing? They are playing volleyball on the beach. There's a man under the umbrella. He's reading a book. Look! A little girl is collecting shells on the beach. She's very happy.

重敲大音叉时，发出的声音____。