在三角形$ABC$中，角$A,B,C$所对的边分别是$a,b,c,$且$a\cos C,b\cos B,c\cos A$成等差数列，若$b=\sqrt{3}$，则$a+c$的最大值为

第二次世界大战全面爆发的标志性事件是（    ）

⑴How does your brother feel? ____     A. They are thirsty.

⑵How does your sister feel? ____        B. He's sad.

⑶How does your dog feel? ____          C. I am cold.

⑷How do they feel? ____                     D. She's tired.

⑸How do you feel? ____                      E. We are cool.

⑹How do we feel?____                        F. It's hungry.