(i) a narcotics indictment returned in any county within such cities may be prosecuted in the special narcotics part to which it is assigned pursuant to section one hundred seventy-seven-b irrespective of the county in which the part is held and in which the crime charged was committed;
(ii) any assistant district attorney appointed pursuant to the plan authorized by section one hundred seventy-seven-c may prosecute all offenses cognizable by any special narcotics part irrespective of the county in which the part is held and in which the crime charged was committed; and
(iii) upon the application of the assistant district attorney in charge of the special narcotics parts appointed pursuant to the plan authorized by section one hundred seventy-seven-c, one or more grand juries may be drawn and impaneled for a special narcotics part upon the order of the justice assigned to such part, which grand jury may exercise all the powers of a grand jury in the county in which it is impaneled and may in addition exercise its powers with respect to the alleged commission of an offense in any county wholly contained in a city having a population of one million or more involving the sale or possession of a narcotic drug and any other offense that could be properly joined therewith in an indictment.