已知两定点F₁，F₂和一动点M，则“|MF₁|+|MF₂|=2a（2a为正常数）”是“点M的轨迹是以F₁，F₂为焦点的椭圆”的（　　）

A．充要条件

B．必要不充分条件

C．充分不必要条件

D．非充分非必要条件

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 are older, they （）.

A．know that Father Christmas is real

B．ask their mother to fill their stockings with presents

C．know that Father Christmas is really their father

D．know that Father Christmas is really their friend