已知▱ABCD，添加下列一个条件：①AC⊥BD②∠BAD=90°③AB=BC④AC=BD，其中能使▱ABCD是菱形的为（ ）
A．①③
B．②③
C．③④
D．①②③
∵四边形ABCD是平行四边形，
①若AC⊥BD，则可得其为菱形，故①选项正确，
②中∠BAD=90°，得到一矩形，不是菱形，所以②错误，
③中一组邻边相等，也可得到一菱形，所以③成立，
④中并不能得到其为矩形，菱形或正方形等，所以④不成立，
故A选项中①③都正确，B中②不成立，C中④错误，而D中多一个选项②也不对，
则能使▱ABCD是菱形的有①或③．
故选：A．
Swans are beautiful birds. You must like their beautiful feathers and beautiful dancing. But how much do you know about them? Do you know that swans belong to the same family as ducks?
Swans are birds of the family Anatidae (鸭科). This family also includes geese (鹅) and ducks. Swans are the largest members of the duck family Anatidae, and are one of the largest flying birds. Swans can fly as fast as 60 miles per hour . Their wingspans can be almost 3 m. The largest swans can reach length of over 1.5 m and weigh over 15 kg. Males are usually bigger and heavier than females.
Swans usually have only one lover for life time. Swans begin breeding(繁殖) between the ages of 3-4. Female swans generally lay five to eight eggs per year, all within days of one another. Their eggs are very big. Some may weigh 400 grams. However, she will not start hatching (孵化) them until the last egg is laid. It takes swans between 35 and 42 days to hatch their eggs .
Swans are beautiful. The swans living in Northern Hemisphere (半球) have white feathers but the ones who live in Southern Hemisphere are black and white. The legs of swans are usually a dark blackish grey colour.
Swans usually live in warm places, always close to some sort of water. Swans feed in the water and on land. They mostly eat plants in the water. Their nests (巢) are on the ground near water and about a metre across. The male helps build the nests.
| An Introduction to Swans | |
| Family | Swans, geese and ducks are of the 小题1: family, Anatidae. |
| Appearance | Size: They are one of the 小题2: flying birds. Her wing span will grow to 1.5 metres 小题3:. 小题4: : They can be as heavy as 15 kg. Colours: The swans living in the 小题5: Hemisphere are white. Those living in the Southern Hemisphere are black and white. They have dark blackish grey legs. Feathers: over 25000. |
| 小题6: | They can dance __小题7:__. They can fly as fast as 60 miles per hour. |
| Lifestyle | 小题8: habitat: They live in warm places near water. Diet: They live on 小题9: plants. Nests: They build nests on the ground near water, about a metre across. Breeding(繁殖): They 小题10: from 5 to 10 eggs that take from 35 to 42 days to hatch. |