Cesium 137 is a radioactive element that constitutes one of the main sources of radioactivity in nuclear reactor waste. The time $T$, in years, during which a cesium 137 atom remains radioactive can be approximated by a random variable $T$ that follows the exponential distribution with parameter $\lambda = \frac { \ln 2 } { 30 }$. What is the probability that a cesium 137 atom remains radioactive for at least 60 years? a. 0.125 b. 0.25 c. 0.75 d. 0.875