[练习题]Summarization

Directions: The paragraph is followed by an incomplete phrase or sentence which summarizes the main idea of the paragraph. Spell out the missing letters of the word.


All plants and animals that have been studied carefully seem to have built-in clocks. These biological clocks, as they are called, usually are not quite exact in measuring time. However, they work pretty well because they are "reset" each day, maybe when the sun comes up.
Biological clocks of plants and animals f pretty well.