Please help me answer this questions

4. [5] A greedy algorithm makes a greedy choice before finding an optimal solution of subproblem. Thus, it solves a problem in top-down approach.

5. [5] The optimal data encoding/decoding achievable by a character code can always be achieved by a variable length code with no ambiguity.

6. [5] Quick sort is much faster than quadratic sorting algorithms such as insertion-sort and selectionsort.