The strength of an electromagnet can be altered by increasing the number of coils around the core. The more times the coil is wrapped, the stronger the electromagnet is.
Your answer is: B) Increasing the number of coils around the core
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The answer to this question would be: vitamin D
The UV is needed by the skin to make previtamin D3. Previtamin D3 or cholecalciferol made from the skin will be changed in the liver into 25- hydroxyvitamin D3 and then sent to the kidney to be changed into 1,25- dihydroxyvitamin D3. The last change in the kidney will active the vitamin D. Vitamin D has a role in the calcium absorption which was will strengthen the bone tissue.
You have to upload this in the area of mathematicians..!
Given that,
Mass of each washer = 4.9 g
We need to calculate the mass of two washers in kg
Using conversion of unit
Mass of each washer 
So, Mass of two washers is
Put the value of m
If 4 washer are attached to the spring
We need to calculate the applied force on the car
Using formula of force
Put the value into the formula
Hence, (i), The mass of two washers is 0.0098 kg.
(ii). The applied force on the car is 0.192 N.
Explanation is in this link
https://doubtnut.app/physics-question/when-the-distance-between-two-charged-particles-is-doubled-the-force-between-them-becomes-422318076
