Which of the following pairs of factors affects the solubility of a particular substance
a. temperature and the nature of solute and solvent
b. temperature and degree of mixing
c. particle size and degree of mixing
d. particle size and temperature

I believe it's A. Temperature and the nature of the solute and the solvent.

The answer is d.potassium

Radon-222 or rn-222 has a half-life of 3.82 days. to determine the fraction of an original sample of rn-222 that remains unchanged after 7.646 days, we calculate the amount of rn-222 remaining after 7.646 days and divide it with the original amount. using 1 gram as a basis and using half-life calculation methods, we can determine that  0.249 or 0.25  grams of rn-222 will remain, and thus the fraction remaining is 0.25 or 1/4.