下列说法正确的是（   ）

F为抛物线$y^2=4x$的焦点，A,B,C为抛物线上三点．O为坐标原点，若F是$△ABC$的重心,$△OFA,△OFB,△OFC$的面积分别为$S_1,S_2,S_3$，则$S_1^2+S_2^2+S_3^2$的值为(    )  

下面措施中，哪一种对保持生态环境最有利?（     ）

:We're going camping in the forest this weekend. What shall we do then?

:Let's go hiking and collect leaves.

:Sounds great! I want to ____stars(星星)at night，too. That's beautiful.

:I'd like to take some ____. There're many flowers in the forest.

:Good idea! But first，we should ____the tent on the grass. And l want to catch fish in the river. Because your mum ____eating fish.

:Thanks，dear! And we can build campfire and cook fish ____our tent.