[1] Entomologists study insects. [2] These scientists have discovered that thousands of species of insects serve as food sources for birds, reptiles, fish, and even plants. [3] Often thought of as pests, most insects are beneficial to humans. [4] Various species of moths, butterflies, wasps, bees, and flies, for example, have pollinated fruit trees, vegetables, and flowering plants. Which revision will correct an inappropriate shift in verb tense?

1. change "study" to "studied" in sentence 1
2. change "serve" to "have served" in sentence 2
3. change "thought" to "having thought" in sentence 3
4. change "have pollinated" to "pollinate" in sentence 4