Python去掉列表中的方括号

图片说明
如图,想把这个列表改成['CC','DD','EE','GG','HH','II','KK','LL','MM']
请教各位大神怎么操作

a = [['cc','dd'],['aa','bb']]

b = [ i for item in a for i in item]
print(b)

xx= [['cc','dd'],['aa','bb']]

yy = [ i for item in xx for i in item]
print(yy)