Listen again to part of the lecture. Then answer the question. Why does the professor say this:
A．To stress the mysterious nature of severe immune responses
B．To argue that anaphylaxis is mainly the result of bad luck
C．To point out that scientists disagree over what causes allergies
D．To suggest that people are crazy to eat offending substances

Walking is the easiest and simplest form. of exercise. By putting one foot in front of the other, you can start to walk your ways towards a healthier lifestyle. Walking is an easy way to challenge the cardiovascular（心脏血管的)system while at the same time building muscles. But to achieve these goals, a little more than a gentle walk around the block is required.
Walking actually bums as many calories as running the same distance—if done in the correct manner and at a reasonable pace.
Start with a basic program that you can complete three or four times a week. This should include five main steps:
1) A beginning warm-up for about 5 minutes.
2) A little bit faster period of 5 minutes at medium level.
3) A 10-minute" burning" at a high level.
4) A 5-minute cool down period at a low level again.
5) A suitable walking pace of around 5 kilometers per hour will bum around 80 calories in 30 minutes.
According to the passage, walking is a simple way to ______.
A．lose weight
B．cure blood diseases
C．avoid heart attacks
D．strengthen heart functions

A．It has a small desk.
B．It's not big enough.
C．It's just right for one person.
D．It's not painted nicely.

What can be inferred about surviving anaphylaxis？
A．It is impossible for children to survive anaphylaxis.
B．Only a few people have ever survived anaphylaxis.
C．Survival depends on immediate medical treatment.
D．The professor knows someone who survived the disease.

I've never seen you __ behind the door.
A．standing
B．stands
C．to stand
D．stood

It was suggested that the agreement __ before the end of the year.
A．be signed
B．was signed
C．would be signed
D．had been signed