singular values of a smaller matrix

[td21]

let B be a matrix formed by deleting n-r rows and n-r columns from A. Why is the singular values of B smaller than those of A?

To be precise: for the r singular values of B compared with the r biggest singular values of A.

i,e. the biggest singular value of B smaller than biggest singular value of A;the second biggest singular value of B smaller than the second biggest singular value of A......