△ABC与△A′B′C′是位似图形，且△ABC与△A′B′C′的位似比是1：2，已知△ABC的面积是2，则△A′B′C′的面积是（　　）

小明用自制的电磁铁来研究它的磁性强弱与哪些因素有关．实验器材：两个相同大小的大铁钉、一些漆包线、开关、电源、滑动变阻器、一些回形针和电流表等．在实验中，用漆包线绕制横截面和紧密程度相同的三个线圈，其中两个线圈插入铁钉，另一个线圈内无铁钉．按如图电路进行实验，实验记录的数据如表

电流表的作用是____ 

分析表中数据，可得影响通电螺线管磁性强弱的因素有：有无铁芯、____ 

    Mike eats breakfast five times a week . He doesn't eat breakfast on Saturday and Sunday . He washes the dishes twice a week . He washes them on Saturday and Sunday . He waters the plants four times a week . He doesn't water them on Monday , Wednesday and Friday . He always sets the table . He sets the table from Sunday to Saturday . He washes his dog once a week . He washes it only on Saturday . He goes swimming twelve times a month . He swims on Monday , Wednesday and Friday . But he never cleans his room .