A mother and her young son get into a bus in a small city and sit down. The bus conductor comes to them for their money. The mother says: "I want one ticket to the zoo " and gives him two yuan. The conductor looks at the small boy for a few seconds  and then says to him, "How old are you ,young man?" The mother begins to speak, but the conductor stops her, The boy says: "I'm four years old at home, and two in the buses." At last the mother has to take one yuan out of her pocket  and gives it to the conductor. At last the mother  _   .
buys another ticket buys a half-price ticket for the boy says sorry to the conductor gives two yuan to the conductor