The personnel department of a large manufacturing firm selected a random sample of 23 workers. The workers were interviewed and given several tests. On the basis of the test results, the following variables were investigated: X₂ = manual dexterity score, X₃ = mental aptitude score, and X₄ = personnel assessment score.

Subsequently, the workers were observed in order to determine the average number of units of work completed (Y) in a given time period for each worker. Regression analysis yielded these results:

Y = -212 + 1.90X₂ + 2.00X₃ + 0.25X₄, R² = .75.

(.050) (.060) (.20)

The quantities in parentheses are the standard errors of the regression coefficients. The standard error of the regression is 25, and the standard deviation of the dependent variable is 50.

Which variables are making a significant contribution to the prediction of units of work completed at the .01 significance level (two tailed)?

A. All three variables.

B. Manual dexterity and personnel assessment.

C. Manual dexterity and mental aptitude.

D. Personnel assessment.

E. None of the variables are statistically significant.