和在一定条件下充分反应后，得到的产物为（   ）

A．CH₃COOC₂H₅和 H₂¹⁸O

B．CH₃CO¹⁸OC₂H₅和 H₂¹⁸O

C．CH₃CO¹⁸OC₂H₅和 H₂¹⁸O

D．CH₃COOC₂H₅和 H₂O

Measures have been taken to clean up the lake,but it will be some time________the situation improves. [ ]

A. although  

B. before  

C. unless  

D. when