i. When TR curve is a positively sloped straight line then MR curve is a horizontal line. Under perfect competition MR curve and demand curve are same and the average revenue remains constant and equals to the price for different levels of output due to which MR is constant and TR increases at increasing rate so the TR curve is positively sloped straight line.

ii. When TR curve is a horizontal straight line, then MR is zero because the units sold is same at every level of output and Marginal Revenue is the additional revenue generated from the sale of an additional unit of output. Therefore, MR curve is also a horizontal straight line and coincides with the output-axis.