【单选题】It is taken as a great contribution made by ______ that languages in the world are classified into different language families according to their genetic relations.
【单选题】有下列结论: (1)命题p:?x∈R,x 2 >0总成立,则命题?p:?x∈R,x 2 ≤0总成立. (2)设 p: x x+2 >0,q: x 2 +x-2>0 ,则p是q的充分不必要条件. (3)命题:若ab=0,则a=0或b=0,其否命题是假命题. (4)非零向量 a 和 b 满足 | a |=| b |=| a - b | ,则 a 与 a + b 的夹角为30°. 其中正确的结论有( ...