2/We can study the ... of birds to understand how they fly.

3/My friends are as interested in football .... I am.

4/.....most new PCs have severl USB slots, older ones often only had on .

5/I need. ..... advice on what to do next.

6/My father bought a/an. .....house.

7/AChicken's beak is its.

8/If you want to get better marks, you must be more. .....

9/This morning the headmaster rushed around the school.

10/The taller the building, the ..... the foundation.

11/A convex lens is. .... in the middle than at edges

12/The ...helps us understand the different properties that matter has in its three states

13/The computers we purchased were. ...

14/Edward Jenner's work with a.... vaccine was the first scientific attempt to control a disease by immunization.

15/The sound of jet plane taking off measures between. ...

16/...... invented the zero , negative number , the decumal system and algebra.

17/These capsules rae safe. There are no ....

18/when I accidently met my professor at the airport, I didn't ...him. He looked so different

19/Don't worry , there's no need to answer ..... I'm not in a hurry.

20/I will write my will at night .The understand word is a/an. ...