Diatoms are algae whose silicon dioxide-containing shells are useful economically as

Diatoms are algae whose silicon dioxide-containing shells are useful economically as
 
  A. filters.
  B. fertilizers.
  C. stabilizers.
  D. thickeners.

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Agar is obtained from
 
  A. bacteria.
  B. algae.
  C. protozoans.
  D. plants.

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Macroscopic algae possess a special structure that acts as an anchor and is commonly called
 
  A. a thallus.
  B. a holdfast.
  C. roots.
  D. the stipe.

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Algae
 
  A. often grow in areas where other forms of life may have difficulty.
  B. are strictly macroscopic organisms.
  C. have a vascular system similar to plants.
  D. are only found in the soil.

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Free-floating, photosynthetic organisms found in marine environments are
 
  A. Bucella.
  B. blue-green algae.
  C. krill.
  D. phytoplankton.