For current to be maximum in a circuit $${ X }_{ l }={ X }_{ C }$$(Resonant Condition) $${ W }_{ l }={ rac { 1 }{ { W }_{ c } }  }$$ $${ W }^{ 2 }= rac { 1 }{ { L }_{ c } } = rac { 1 }{ 2ast 18ast { 10 }^{ -6 } } = rac { { 10 }^{ 6 } }{ 36 } $$ $$W= rac { { 10 }^{ 3 } }{ 6 } =2pi f$$ $$f= rac { 1000 }{ 6ast 2pi  } =27Hz$$ $$Max.current= rac { 20 }{ 10ast { 10 }^{ 3 } } =2mA.$$