A stone is tied to one end of string. holding to the other end , the string is whirled in the horizontal plane with progressively increasng speed it breaks at some speed bexause

Breaks because it doesn't have force in the radial direction and thus due to inertia the stones will fly off tangentially.

Explanation:

Until the stone is tied to a string, it will possess centripetal force and centrifugal force along the radial direction of motion due to the tension present in the string. Now, when the rope breaks, it breaks because it doesn't have force in the radial direction and thus due to inertia the stones will fly off tangentially.

electromagnetic shielding is the practice of reducing the electromagnetic field in a space by blocking the field with barriers made of conductive or magnetic materials. shielding is typically applied to enclosures to isolate electrical devices from their surroundings, and to cables to isolate wires from the environment through which the cable runs. electromagnetic shielding that blocks radio frequency electromagnetic radiation is also known as rf shielding.

the shielding can reduce the coupling of radio waves, electromagnetic fields and electrostatic fields. a conductive enclosure used to block electrostatic fields is also known as a faraday cage. the amount of reduction depends very much upon the material used, its thickness, the size of the shielded volume and the frequency of the fields of interest and the size, shape and orientation of apertures in a shield to an incident electromagnetic

from wikipedia

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.