WAEC - Biology (1998 - No. 46)

Phytoplankton are plant-like, microscopic organisms, specifically algae and some bacteria, that live in water and perform photosynthesis, similar to land plants. Hence, they are the primary producers. However, the herbivore will feed on them. The tadpole being placed directly after the phytoplankton means it will be the herbivore (the primary consumer).