In a first difference (FD) regression of y on x, the estimated coefficient on x should be interpreted as a.telling us how variation in x around its unit-specific mean generates changes in y around its unit-specific mean. b.telling us how permanent changes in x generate permanent changes in y. c.telling us how changes in lags of x generate future changes in y. d.telling us how periodical changes of x generate periodical changes in y. e.None of the listed answers is correct