Which ecosystems do not have
symbiotic relationships?
a. deserts
b. all ecosystems contain symbiotic relationships.
c tropical rainforests

The correct option is B. All ecosystems contain symbiotic relationships.

Explanation:

Symbiotic relationship can be described as a relationship between the organisms of two species. It might be harmful for both the species, beneficial for both the species or just beneficial to one of the species. All ecosystems will have some kind of symbiotic relationships. Hence, option B is correct.

Other options like Option A is false because the desert ecosystem does has symbiotic relationships. For example, the Gobi woodpecker lives on a cactus in a desert. In return it eats the insects and other parasites which harm the cactus.

relative dating involves things like higher layers are most recent than rocks lower in the sequence. this can be extended to the known sequence of fossils. rocks containing rabbit fossils are later than those containing dinosaur fossils.  

absolute dating methods mainly use radioactive elements that have known rates of decay and produce recognizable products. for example potassium 40 decays to produce argon. once a rock has solidified the argon is trapped. by measuring the argon and potassium0 you can get an accurate number for the age.  

some other methods are good for archaeological ages. dendrochronology involves recognizing the sequence of tree rings. the orientation of magnetized particles in baked clay can be used to date a fire.

hope this .. .