An econometric study for the period 1960-2004 relates production costs in USA (y) and time (x) such that /=1 (1960), /=2 (1962), and ... /=23 (2004). The following exponential model is obtained: log(yz) = 95.3 + 0.0253t (4.15) (0.008) a- Interpret the estimated slope coefficient. b- Test the null hypothesis that the true value for the slope coefficient is zero at a 5% significance level. c- Ted the above but at a 1% significance level. Why this second hypothesis test is more informative than the first one? Note : The values 4.15 and 0.008 are the standard errors of the respective coefficients