The following table shows the currently accepted half-life values for six parent isotopes. Parent Isotope Currently Accepted Half-Life Values Uranium-238 4.5 billion years Uranium-235 704 million years Thorium-232 14.0 billion years Rubidium-87 48.8 billion years Potassium-40 1.25 billion years Samarium-147 106 billion years Earth is approximately 4.5 billion years old. Given this information and the data in the table, which of the following isotopes could today contain less than 50 percent of its parent isotope? Potassium-40 Rubidium-87 Samarium-147 Uranium-238