Uranium-235, with a half-life of 713,000,000 years, decays to lead-207. If a rock sample is determined to have one-quarter of the uranium-235 content it had when it formed, the age of the rock sample can be estimated to be approximately ________ years old.
A) 1.4 billion.
B) 713 million.
C) 178 billion.
D) 178 million.
E) 28.5 billion.