Bach died in 1750, but it was not until the early 19th century____ his musical gift was fully recognized.
A:while
B:though
C:that
D:after
若x1、x2是方程x2+2(m﹣2)x+m2+4=0的两个实数根,且x12+x22﹣x1x2=21,求m的值.