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.

在除灰系统运行中，提升机与来渣机的启动顺序是（）。

A.先启动提升机，再启动来渣机；

B.先启动来渣机，再启动提升机；

C.同时启动提升机与来渣机；

D.以上都不正确。