Magnetic field lines, generated by a long straight wire carrying a current, are circles concentric with the wire and lie in planes perpendicular to the wire.
a. true
b. false

True

Explanation:

It is true that magnetic field lines around a current carrying wire is in the form of concentric circles and lie in the plane perpendicular to the wire.

This Fact can be Proven by placing the iron filling around the wire and they will be arranged like concentric circles and Magnetic field intensity diminishes as we go away from the wire.  

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.

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