The risky efficient frontier assuming one cannot borrow or lend at the risk$-$ free rate is defined as:

$($A$)$The set of portfolios offering the highest expected return for a given level of standard deviation

$($B$)$The set of portfolios offering the lowest risk for a given expected return

$($C$)$he slope of the tangent equal infinity at the point of the risky efficient frontier of the lowest risky portfolio

$($D$)$The slope of the tangent is zero at the point of the risky efficient frontier which represents the maximum risky portfolio

$($E$)$All the above