According to some experts, carbon dioxide is
accumulating in the atmosphere so rapidly, largely
because of the burning of fossil fuels, that should
the present rate of buildup continue, the global
concentration of this poison will double by the end
of the next century.

(A) that should the present rate of buildup continue,
the global concentration of this poison
(B) should the present rate of buildup continue, the
global concentration of this poison
(C) should the present rate of buildup be continued,
the global concentration of this poison
(D) the global concentration of this poison, were
the present rate of buildup continue,
(E) that the global concentration of this poison, if
the present rate of buildup did continue,

OA D...Please explain