A.夾點(diǎn)之上NH≦NC,夾點(diǎn)之下NH≦NC B.夾點(diǎn)之上NH≦NC,夾點(diǎn)之下NH≧NC C.夾點(diǎn)之上NH≧NC,夾點(diǎn)之下NH≧NC D.夾點(diǎn)之上NH≧NC,夾點(diǎn)之下NH≦NC