 Complete the following phrase:  The towers are the second stage in the salt production process, where the brine concentration is gradually increased. The smallest concentration occurs at tower No. I 9; the brine concentration increases at tower No. III 16 and becomes greatest at tower No. II 30. From the latter, the brine flows in pipelines to the salt-brewing plant the third stage of salt production where salt, sludge and therapeutic lye are produced. The first stage in the process of salt production is pumping brine from the source No. 11 Grzybek fountain. The graduation towers also act as a giant air filter. In 1996, radioactive caesium isotopes Cs-134 and Cs-137 from the 