RC4切丝机的刀门高度变化通过（）变化而实现。

A、进刀气缸

B、刀门气缸

C、高度气缸

D、气缸离合器

C

On Christmas Eve—the night before Christmas Day—children all over Britain put a stocking at the end of their beds before they go to sleep. Their parents usually tell them that Father Christmas will come during the night.

Father Christmas is very kind-hearted. He gets to the top of each house and climbs down the chimney into the fireplace. He fills each of the stockings with Christmas presents.

Of course, Father Christmas isn’t real. In Jim and Kate’s house, "Father Christmas" is really Mr. Green. Mr. Green doesn’t climb down the chimney. He waits until the children are asleep. Then he quietly goes into their bedrooms and fills their stockings with small presents. When they were very young, Mr. Green sometimes wore a red coat. But he doesn’t do that now. The children are no longer young, and they know who "Father Christmas" really is. But they still put their stockings at the end of their beds.

When the children were very young, （）.

A．they didn’t know who Father Christmas was

B．they knew that Father Christmas wasn’t real

C．they thought their father was Father Christmas

D．they knew who put the presents into their stockings