Please answer the following question: Read the following article and select the best answer. Article: Marry is eight years old now. This autumn she begins to go to school. She likes her teachers and has some friends. She studies hard and is good at her classes. So we think she's a good girl. It's Sunday today. It's fine and after breakfast the girl and her parents are going to the zoo. She likes to watch the animals very much. Now they're standing at the bus stop and waiting for the bus. She sees her friend Jim and his grandpa there. "Good morning, grandpa!" says the girl. "Are you going to the zoo, too?" "Yes, we are," says the old man. Then he says to Mary's father, "You have a polite daughter. How old is she?" "Eight, grandpa." answers the girl. "Eight?" the old man says with a smile. "But you're shorter than my stick!" "How old is your stick, then?" says the girl. Question: Mary and her parents are going to the zoo by _ . - car - boat - bus - bike
A:
bus