suppose you have a rock sample containing uranium, some of which is 235u, which decays to 207pb with a half-life of 713 million years. you find that only 25% of the original 235u is still present, the rest having decayed to 207pb. what can you determine about the age of the rock? group of answer choices the rock is less than 713 million years old. the rock is 713 million years old. the rock is 1.4 billion years old. the rock is 2.1 billion years old. the age of the rock cannot be determined.