2  He won't leave Beijing until this message arrives.

By the time this message arrives, he will have left Beijing.

3  They will complete the new motorway by next June.

They won't have completed the new motorway until next June.

Special difficulties  难点

A  Explain the meaning of since in these sentences:

1  Some starting data has to be guessed, since ground stations and satellites cannot see everywhere.

2  John feels much happier since he changed his job．

3  Susan left in July and we haven't seen her since.

4  It hasn't stopped raining since eight o'clock this morning.

5  Since you're so clever, see if you can solve the problem!

B  The computer will be unable to predict whether Princeton, New Jersey, will have sun or rain on a day one month away．(11.13-14)

Supply if or whether; note the sentences where we can use both if and whether.

1  _____ he has signed the contract (or not) doesn't matter.

2  The question is ______ he has signed the contract.

3  I want to know _______ he has signed the contract.

4  I'm concerned about ______ he has signed the contract.

5  Do you know ______ she's arriving by bus or by car?

Multiple choice questions  多项选择题

Choose the correct answers to the following questions.

Comprehension  理解

1  The reason it's hard to forecast the weather is that ______.

a．conditions rapidly deteriorate

b．there is always turbulence

c．only satellites have a panoramic view of conditions

d．the effect of tiny changes cannot be detected or calculated

2  Even with grid points which are sixty miles apart, forecasters  _____.

a．have to depend on satellites for information

b．have to make assumptions about some of the data

c．work on modern weather models

d．prevent errors from multiplying

3  Which of these statements is true?

a．The only way to solve the problem is to have sensors spaced at one-foot intervals.

b．The only way to solve the problem is to have infinitely powerful computers.

c．The only way to solve the problem is to have perfectly accurate readings.

d．At present, there is no way of making absolutely accurate predictions.

4  Tiny variations in temperature, pressure and humidity _____.

a．will be picked up by sensors at one-minute intervals

b．are used to predict the weather one month ahead

c．can lead to completely unexpected weather conditions

d．produce expected results

Structure  句型

5  Generally speaking, any prediction ______ rapidly．(1.2)

a．is deteriorating

b．has deteriorated

c．will deteriorate

d．had deteriorated

6  Only satellites _______ see continent-size features．(11.5-6)

a．are able to

b．could

c．are possible

d．are capable

7  _____ the earth could be covered with sensors．(11.9-10)