A laboratory prepares equal initial activities of two pure isotopes, P and Q. Isotope P has a half-life of 1.0day, whereas isotope Q has a half-life of 4.0days. After 2.0days, what is the ratio of the activity of isotope P to that of isotope Q, AP/AQ?
(a) 0.50
(b) 0.35
(c) 0.71
(d) 0.25