已知点，B为椭圆+=1的左准线与轴的交点，若线段AB的中点C在椭圆上，则该椭圆的离心率为
My parents and I_________ trees last Sunday.（ ）
A．plantB．will plant
C．are plantingD．planted
—Peter，what are you doing？
—Oh，I_________a report about national heroes.（ ）
A．will writeB．am writing
C．wroteD．have written
The teacher is glad to see that Tony is_______than before.（ ）
A．carefulB．more careful
C．most carefulD．the most careful
The doctors worked for ten hours，___________nobody took a break.（ ）
A．soB．forC．butD．or
—Sam，_________I join you in the community service？
—Of course you can.（ ）
A．canB．mustC．shouldD．need
，B为椭圆
+
=1
的左准线与
轴的交点，若线段AB的中点C在椭圆上，则该椭圆的离心率为 
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