math help please

Devise the exponential growth function that fits the given data, then answer the accompanying question. Be sure to identify the reference point (t = 0) and units of time. Uranium-238 (U-238) has a half-life of 4.5 billion years. Geologists find a rock containing a mixture of U-238 and lead, and determine that 66% of the original U-238 remains; the other 34% has decayed into lead. How old is the rock? What is the reference point (t = 0)? O A. When the rock contained 100% of the original amount of U-238. O B. When the rock does not contain any more of the original amount of U-238. O C. The state of the rock 4.5 billions years ago. O D. The current state of the rock with 66% of the original amount of U-238. What are the units of time? O months O billions of years O years O decades Write the exponential growth function. Use yo the initial value of y. y(t) = (Type an expression. Round coefficients to three decimal places as needed.) How old is the rock