Which type of lens is shown in the picture below?
plane
refractional
concave

because growing process had stoped after 4-5 year

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.