When does a comet's bright tail appear?

a.
when it nears the moon

b.
when it nears another comet

c.
when it collides with a planet

d.
when it nears the sun

Which element forms the basis of all organic organisms a carbon b oxygen c hydrogen d nitrogen

Which soil would most likely be found in the arctic? andisols gelisols histosols spodosols

After a group of researchers conducts a new experiment that has never been conducted before, what is the best way for other scientists to make sure that the data is real