Which of these fits gap 21?

Directions: Complete this passage by selecting the most suitable option for the corresponding gap.
Radium is a radioactive element. Let us see what “radioactive” _____ (21) ____. Radioactive elements do not change from year to year. But a few of the heaviest atoms break down and change into other kinds. This breakdown or ____ (22) _____ is called “radio activity”. Each radioactive element disintegrates by giving off rays at a certain rate. This rate cannot be _____ (23) ____ or slowed down by any known method. Some changes occur rapidly, others slowly, but in all cases the action cannot be _____ (24) ______ by man.