Why must there be a chain at the bottom of a truck​

Known as automatic tire chains and sometimes referred to by brand names like Onspot or Insta-Chain, these chains essentially hang listlessly from the vehicle's suspension until inclement weather arrives. When that happens, a driver can flip a switch that will lower the chains so they hang in front of the back tires.

Explanation:

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what was the answer?

adding a(n) to an electric circuit would increase the magnetic field around a wire;

b. solenoid

a solenoid is a type of electromagnet whose purpose is to generate a controlled magnetic field.

if the purpose of the solenoid is instead to impede changes in the electric current, a solenoid can be more specifically classified as an inductor rather than an electromagnet.