State whether the following statements are true or false.
$(a)$ The magnetic field at the centre of a long circular coil carrying current consists of parallel straight lines.
$(b)$ $A$ wire with green insulation is usually the live wire of an electric supply.

• A
  $(a)-(T); (b)-(T)$
• B
  $(a)-(F); (b)-(T)$
• C
  $(a)-(F); (b)-(F)$
• D
  $(a)-(T); (b)-(F)$