Plant guard cells(保卫细胞) have a higher [C1-] than the outside environment. The most likely explanation for this is( ).

A． Water leaves the cell by osmosis.

B． C1- is diffusing through transmembrane channels.

C． C1- diffusion is aided by a mobile carrier.

D．An active transport symport transports C1-/H+.

E．None of the above

简要叙述真核生物mRNA的转录后加工的方式，这些加工方式各有何意义