Jim is from New York(纽约). His home is near the school. So he often goes to school by bike. Every day it takes him about fifteen minutes to get there. He always gets to school on time(准时).

    Mary is a student of a middle school in Tokyo（东京）. The school is far from her home. Every day she has to go to school by subway. It takes her about forty minutes, she often gets up very early because she doesn't want to be late for school. She is a good student.

    Jenny's home is in London. It is far from her school. She usually goes to school by bus. This kind of bus has two floors. It takes her an hour every day. She must get up very early every morning. She often gets there early.

    Tom is a student of a middle school in Iceland（冰岛）. There is snow all year around. So he usually goes to school by sledge（雪橇）. It takes about twenty minutes to get there. He is sometimes late for school.

—He spoke ______ fast _______ I didn't follow him.

某氧原子的质最为ag，¹²C原子的质量为bg，且N_A表示阿伏加德罗常数，下列说法中正确的是（   ）