A geologist wishes to determine the age of a rock believed to be about 3billion years old. Which radioactive isotope is the most suitable for this purpose?
(a) 14C (half-life =5.73×103yr)
(b) 238U (half-life =4.5×109yr)
(c) 40K (half-life =1.25×109yr)
(d) 235U (half-life =7.1×108yr)