（A） sang more than 20 leading roles at the Metropolitan Opra since having
（B） sang more than 20 leading roles at the Metropolitan Opra since having had
（C） has sung more than 20 leading roles at the Metropolitan Opra since
（D） has sung more than 20 leading roles at the Metropolitan Opra after
（E） has sung more than 20 leading roles at the Metropolitan Opra subsequently to
17.Even though its per capita food supply hardly increased during two decades， stringent rationing and planned distribution have allowed the People's Republic of China to ensure nutritional levels of 2，000 calories per person per day for its population.
（A） Even though its per capita food supply hardly increased during
（B） Even though its per capita food supply has hardly increased in
（C） Despite its per capita food supply hardly increasing over
（D） Despite there being hardly any increase in its per capita food supply during
（E） Although there is hardly any increase in per capita food supply for
18.Although the Supreme Court ruled as long ago as 1880 that Blacks could not be excluded outright from jury service， nearly a century of case-by-case adjudication has been necessary to develop and enforce the principle that all juries must be drawn from“a fair cross section of the community.”
（A） has been necessary to develop and enforce the principle that all juries must be
（B） was necessary for developing and enforcing the principle of all juries being
（C） was to be necessary in developing and enforcing the principle of all juries to be
（D） is necessary to develop and enforce the principle that all juries must be
（E） will be necessary for developing and enforcing the principle of all juries being
