A communications firm employs a large number of technical sales representatives who make personal visits to potential customers. For data collected on y = average amount of sales per call and x = number of years of experience, the accompanying scatter diagram has been constructed. If a third-order polynomial model were fitted to these data, what would be the signs of the partial regression coefficients in the model?

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600 T 500 400 300 200 100 12 10 Years of Experience 5 15 14