原句:
Studies ( by Hargrave and Geen) estimated natural community grazing rates by measuring feeding rates of individual zooplankton species in the laboratory and then computing community grazing rates for field conditions using the known population density of grazers.
杨鹏长难句的解释中说, and then后面省略了一个 by,请问在GMAT中这个by是否可以省略呢?