For the $0/1$ knapsack problem

$($a$)$ write a greedy algorithm as an approximation algorithm for the problem

$($b$)$ what is the approximation ratio of the greedy algorithm in $($a$)?$

$($c$)$ What is the time efficiency class of the algorithm in $($a$)?$

$($d$)$ Can you modify the greedy algorithm in $($a$)$ to provide approximation algorithm with a better

approximation ratio?