SAT Math Practice Question

The world's population has grown at an average rate of 1.9 percent per year since 1945. There were approximately 4 billion people in the world in 1975. Which of the following functions represents the world's population $P$, in billions of people, $t$ years since $1975 ?(1$ billion $=1,000,000,000)$

1. A.$P(t)=4(1.019)^{t}$
2. B.$P(t)=4(1.9)^{t}$
3. C.$P(t)=1.19 t+4$
4. D.$P(t)=1.019 t+4$