An algorithm designed to track a walking distance goal of 50 units is shown in Figure 1.

Figure 1

1  limit ← 50
2  current ← 0
3  WHILE current < limit DO
4      step ← USERINPUT
5      IF step < 0 THEN
6          OUTPUT 'Invalid step'
7      ELSE
8          current ← current + step
9      ENDIF
10 ENDWHILE

Select the option that correctly identifies the programming concept implemented on line 3.