In a meta-analysis of 100 studies that compared boys' and girls' knowledge of AIDS, an average ES of 0.50 was found. This number represents:. a) a correlation of 0.50 b) a mean square (variance) of 0.50 c) a t ratio of 0.50 d) a difference of 0.50 standard deviation units

the answer is the first electromagnet (a). this is because it has more coils around the ferromagnetic material. increasing the number of coils increases the surface area in which the metal interacts with the current in the coil. this enables more atoms in the metal to become aligned hence becoming more magnetized.