Suppose that the world's current oil reserves is R = 2190 billion barrels. If, on average, the total reserves is decreasing by 21 billion barrels of oil each year, answer the following: A.) Give a linear equation for the total remaining oil reserves, R, in billions of barrels, in terms of t, the number of years since now. (Be sure to use the correct variable and Preview before you submit.) R= B.) 15 years from now, the total oil reserves will be billions of barrels. C.) If no other oil is deposited into the reserves, the world's oil reserves will be completely depleted (all used up) approximately years from now. (Round your answer to two decimal places.)