I’m going to spend my summer holiday in Hawaii.—What a lucky boy! And don’t forget to send me a postcard.

How often do you eat out —_____, but usually once a week.

依次填入下面一段文字横线处的句子，衔接最恰当的一组是一棵树就是一条竖立着的河流， ， ，树皮交错的纹路形如流水，从根部到梢头，就像从源头到下游，只是有些平静些， ；有些粗粝些， 。①重现风雨波澜②枝枝杈杈是分支流③树干是主河道④像水一样光滑

_____ of the workers in this factory is about five hundred. ______ of them are women.

Scientists from different countries are trying to_______ a good way to solve the energy problem.