[a] What is Martha's marginal benefit of the fifth unit of good X? $19 (b) Martha is currently consuming 4 units of X and 2 units of Y. Use marginal analysis to explain why this combination is not optimal for Martha. The total benefit combination is not efficient due to the level Y is at (c) What is Martha's optimal combination of goods X and Y? 3 units X 4 units Y [d] Indicate whether each of the following will cause the optimal quantity of good Y to increase, decrease, or stay the same. (i) The price of good Y doubles optimal quality will increase [ii] Martha's income falls to $10 with no changes in prices. optimal quality increases [iii] Martha's income doubles, and the price of both goods double optimal quality goes down