It was difficult __________ something because it was dark.
A:see
B:to see
C:sees
D:seeing
I think roses are the ____ (nice) of all the flowers.
24mL浓度为0.05mol・L﹣1的Na2SO3溶液,恰好与20mL浓度为0.02mol・L﹣1的K2Cr2O7溶液完全反应,已知Na2SO3能被K2Cr2O7氧化为Na2SO4,则元素Cr在还原产物中的化合价为( )