A:He is reading a book.
B:He thinks the book is difficult.
C:He doesn't like the book.
①N2+O2 放电_ 2NO
②4AgNO3 △_ 2Ag2O+4NO2↑+O2↑
③2NH4Cl+Ba(OH)2 △_ BaCl2+2NH3↑+2H2O.
若集合M={a,b,c}中的元素是△ABC的三边长,则△ABC一定不是( )