A:乙烯的结构简式CH2CH2
B:Na2O2中氧元素的化合价为-2
C:Cl-的结构示意图:
D:乙酸的结构简式:CH3COOH
⑴____ A. teacher B. great C. please D. eat
⑵____ A. school B. good C. book D. football
⑶____ A. cousin B. about C. trousers D. out
⑷____ A. window B. flower C. now D. down
⑸____ A. her B. teacher C. fattier D. flower
生态平衡是指生态系统中各种生物( )