Lichens take up nutrients and pollutants from airborne dust, so many cannot survive where the level ...

Lichens take up nutrients and pollutants from airborne dust, so many cannot survive where the level of air pollution is high. The U.S. Forest Service monitors lichens on tree trunks as part of its program to assess the health of forest communities. Lichen is a(n) ____.
 
  a. pioneer species
  b. indicator species
  c. primary succession species
  d. keystone species
  e. exotic species

Question 2

Membrane potential cannot exceed +30 mV, because when it reaches this level, gated sodium channels close, and:
 
  a. potassium only moves across the cell membrane via passive transport proteins
  b. sodium only moves across the cell membrane via passive transport proteins
  c. potassium gated channels open
  d. potassium flows into the cytoplasm
  e. the action potential is not propagated