A given length of a wire carries a steady current. It is bent first to form a circular plane coil of one turn. If a loop of same length is now bent more sharply to give a double loop of smaller radius, the magnetic field at the centre caused by the same current is :

• A

  a quarter of its first value

• B

  unaltered

• C

  four times of its first value

• D

  half of its first value