Fill in the Blanks in the Following Table A If Activity X Is Increased from 3 to 4

Fill in the blanks in the following table. Then use the table to answer the questions:
$\begin{array}{ccccc}&\text { Total } & \text { Total } & \text { Marginal } & \text { Marginal } & \text { Net } \\X&\text { Benefit } & \text { Cost } & \text { Benefit } & \text { Cost } & \text { Benefit }\\\hline0&-&0&xx&xx&0\\1&200&30&-&-&-&\\2&-&-&100&-&230\\3&-&-&-&50&270\\4&450&-&-&65&-\\5&-&255&30&-&-\\6&500&-&-&75&-\end{array}$
a. If activity X is increased from 3 to 4 units, total benefit ___________ by $_________.
b. If X is increased from 3 to 4 units, total cost ___________ by $_________.
c. If X is decreased from 6 to 5 units, total benefit ___________ by $_________, and total cost ___________ by $_________. Thus, net benefit _____________.
d. The optimal level of activity is ___ units of activity X, and net benefit at the optimal level of X is $____________.