I regret__________ you that we are unable to offer you employment.
A. to inform
B. informing
C. to have.Inforemed
D. informed
Between l974 and 1999,the nulnber of overseas visitors expanded__________28%.
A. for
B. by
C. to
D. from
若信号满足关系式y(t)=k·x(t),(式中k为常数),则其互相关系数为______。