Assignment problem — solving a $4\times4$ cost matrix

Four workers are required to be assigned to 4 tasks. The number of hours required to complete the tasks by each of the workers is given in the following table:

$$\begin{array}{c|cccc} & W_1&W_2&W_3&W_4\\\hline T_1 & 20&30&40&50\\ T_2 & 40&50&60&70\\ T_3 & 70&80&90&80\\ T_4 & 30&50&80&40 \end{array}$$

Then, which of the following is true?