B

Quan ho Bac Ninh singing is one of (1) ------the mostpopularthe best well-known social customs appearing widely in the northern provinces of Bac Ninh and Bac Giang. Quan ho folk songs are alternating response songs between the groups of (2) ------student and teachermother and fathermale and female. A group of female from one village sings with a group of male from another village with similar (3) ------melodieswordsnames, but different lyrics, and always with alternating tunes.Quan ho folk songs have 213 different melody variations and more than 400 song lyrics. A song lyric includes two (4) ------songspartsparagraphs : the principal text is the core of the song, containing its base lyrics. The secondary text includes words that are added to the melodies, such as i hi, u hu, a ha..In 2009, Quan ho Bac Ninh folk songs were (5) ------consideredrecognisedconstructed as the Intangible Cultural Heritage of Humanity by the UNESCO.

ta có : 3/a+b=2/b+c=1/c+a=>a+b/3=b+c/2=c+a/1

=a+b-b-c/3-2=a-c/1

=>c+a=a-c=>c=0=>b=2a

thay c=0;b=2a vào M ta đc:

M=2a+3.2a+2020.0/3a+2.2a-2021.0=8a/7a=8/7

Cho bảng "tần số" : 

Từ bảng "tần số" trên hãy lập một bảng thống kê ban đầu .

Hãy giải thích vì sao trong thời kì thuộc Pháp , đồng bào ở các dân tộc ở Việt Bắc và Tây Nguyên phải đốt cỏ tranh lấy tro để ăn