Read the following passage, and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions.

Plants and animals will find it difficult to escape from or adjust to the effects of global warming. Scientists  have  already  observed  shifts  in  the  lifecycles  of  many  plants  and animals, such as flowers blooming earlier and birds hatching earlier in the spring. Many species have begun  shifting where  they live or  their annual  migration  patterns  due to warmer temperatures. With further warming, animals will tend to migrate toward the poles and up mountainsides toward higher elevations. Plants will also attempt to shift their ranges, seeking new areas as old habitats grow too warm. In many places, however, human development will prevent these shifts. Species that find cities or farmland blocking their way north or south may become extinct. Species living in unique ecosystems, such as  those found in polar and mountaintop regions, are  especially at  risk because  migration  to  new habitats  is not possible. For example, polar bears and marine mammals in the Arctic are  already threatened by dwindling sea ice but have nowhere farther north to go. Projecting species extinction due to global warming is extremely difficult. Some scientists have estimated that 20 to 50 percent of species could be committed to extinction with 2 to 3 Celsius degrees of further warming.  The rate  of warming, not just  the magnitude,  is extremely important for plants and animals. Some species and even entire ecosystems, such as certain types of forest, may not be able to adjust quickly enough and may disappear. Ocean ecosystems, especially fragile ones like coral reefs, will also be affected by global warming.  Warmer ocean  temperatures  can  cause  coral  to  “bleach”,  a  state  which  if prolonged will lead to the death of the coral. Scientists estimate that even 1 Celsius degree of additional warming could lead to widespread bleaching and death of coral reefs around the world. Also, increasing  carbon dioxide in the atmosphere  enters the ocean and increases the acidity of ocean waters. This acidification further stresses ocean ecosystems.

Scientists have observed that warmer temperatures in the spring cause flowers to_________.