Identify two structural features of purines and pyrimidines.

purines

__contain two heterocyclic rings.
__contain only two ring nitrogen atoms.
__contain four ring nitrogen atoms.
__contain one heterocyclic ring.
__contain only three ring nitrogen atoms.

pyrimidines

__contain one heterocyclic ring.
__contain four ring nitrogen atoms.
__contain only three ring nitrogen atoms.
__contain two heterocyclic rings.
__contain only two ring nitrogen atoms.

Tara sees a commercial for a brand of pills that promise to boost energy and people perform better in work and school. as people in the commercial talk about how much better they feel when taking the pill, tara notices a message on the bottom of the screen that reads, "this pill is not approved by doctors and has not been tested in controlled experiments." what is the best conclusion that tara should make about the commercial? it is making scientific claims based on subjective observations. it is making scientific claims based on opinions instead of pseudoscientific facts. it is making pseudoscientific claims based on objective observations. it is making pseudoscientific claims based on opinions instead of scientific facts.

Dermal tissue in plants stores 1.extra food 2.transports water and nutrients 3.transports waste materials 4.is similar to epithelial cells in animals

The concept of keystone species is controversial among ecologists because most organisms are highly interdependent. if each of the trophic levels is dependant on all others how can we say one is most important