Inclusion probabilities — they always sum to the expected sample size

Consider a population of 50 units, $\{1,2,3,\dots,50\}$ and suppose that 50 possible samples are listed as $\{1\}$, $\{1,2\}$, $\{1,2,3\}$, …, $\{1,2,3,\dots,50\}$. One of these samples is choosen at random. Let $\pi_i$ be the probability that unit $i$ of the population is in the selected sample. Then which of the following is necessarily true?