Each firm knows where its MR = MC output level is located

Using the table, Johnny's marginal cost of the 600th slice of pizza is

A) $3.50. B) $12.50. C) zero. D) $1.50.

Suppose an economics forecaster discovers that on days when the sunspot count is high stock market on the following day is bullish, that is stock market prices edge upwards

In addition, he also observes that on days with a low sunspot count the following day the stock market tends to be bearish, that is stock market prices tend to fall. The forecaster then concludes that there is a positive relationship between the sunspot count and stock market prices and proceeds to base his investment decisions on this premise. What kind of an error has this forecaster made?