在$△ABC$中，$a,b,c$是角$A,B,C$的对边，若$a,b,c$成等比数列，$A=45°$，则$\frac{b\sin B}{c}=$(    )

    It is the world's most well-known and popular song. It has been sung hundreds of millions of times each year. It has been heard in outer space, at baseball stadiums, in movies and in domiciles(住处)from the White House to the most remote(遥远的) backwoods shack(窝棚) in Misissippi. It has been wired into millions of music boxes, greeting cards, watches, toys, games and other tune-playing products. And it is the only song in the English language to make people of all ages smile and sing along. The song, of course, is Happy Birthday to You".

    This amazing song began its life in 1893. Written by two sisters from Kentucky, Mildred and Patty Hill, it was originally (起初) named “Good Morning to All”, and intended as a classroom greeting from teachers to students:

    Good morning to you

    Good morning to you

    Good morning, dear children

    Good morning to all.

    To this day, no one knows for sure who wrote the new words to Hill's melody(旋律),or exactly when it happened. “Happy Birthday to You” first appeared in a songbook in1924.The words were presented as the second stanza(节) to "Good morning to You".