Which one of the following is true $?$ A$,$ CFDs are over$-$ the$-$counter instruments that can be used to gain exposure to equities,bonds,currencies,futures and options. B$,$ CFDs are over$-$the$-$counter instruments that can be used to gain exposure to equities $,$bonds,currencies,,indices and commodities. C$,$ CFDs are exchange$-$traded instruments that can be used to gain exposure to equities $,$bonds,currencies,,futures and options. D$,$ CFDs are exchange$-$traded instruments that can be used to gain exposure to equities,bonds,currencies,indices and commodities. $?$