2×4____45÷9 25____4×7 25÷5____5×1
72÷9____6 63÷7____3×3 54÷9____2×4
This is ________ useful book. Let's buy it.
根据图中通电螺线管的N极,标出磁感线方向、小磁针的N极,并在括号内标出电源的正、负极.