In a snowy corridor, the time interval separating two successive avalanches, called the occurrence time of an avalanche, expressed in years, is modelled by a random variable $T$ which follows an exponential distribution. It has been established that an avalanche is triggered on average every 5 years. Thus $E ( T ) = 5$. The probability $P ( T \geqslant 5 )$ is equal to: a. 0.5 b. $1 - \mathrm{e}^{-1}$ c. $\mathrm{e}^{-1}$ d. $\mathrm{e}^{-25}$