[1] holding everything else fixed, a one-day increase in labor per hectare is associated with a 0.2033 kg change in yield

[2] all the wave indicators have vifs > 5 so collinearity is a concern
    but since we typically don't interpret coefficients on controls, and collinearity is a problem of precision, it's not a big deal here

[3.1] see graph

[3.2] yes, there is evidence of heteroskedasticity because the spread widens as predicted values increase

[4.1] 0.0458 (naive), 0.0855 (robust), 0.0977 (clustered)

[4.2] clustered is most appropriate because it takes care of heteroskedasticity and within-cluster correlation

[5] since the coefficient on the interaction is positive and significant
    it suggests fertilizer is more effective the more labor you have