下列性状中属于相对性状的是（　　）

-- Who is your English teacher, Helen?

-- __________ is.

    Hello, everyone! I am your1 today. First, I'd like to 2 you something about London before we3London is a very big city of England. The people here are friendly. They are always 4 to tell visitors something about the city.

    London is a busy place. People_5hard. They are very 6 all the time (一直). London is also a very beautiful city. It has a lot of parks and gardens (花园). When you are in London, you can7the British Museum. It can tell you a lot of 8stories. You may also visit9 place. It is the River Thames. It's long and wide. There are many boats on the river.

    May you have a good10！We will get to London in two hours. Now let's have a guessing game.

____ 重力                    A提水时，水桶对手的拉力

____ 弹力                    B橡皮筋被拉长时产生的力

____ 反冲力                C喷气式飞机考喷气发动机产生的力​