Why does adenine always pair with thymine and guanine with cytosine in dna?

Melting glaciers is a serious threat to the environment what is the possible consequence of melting glaciers on polar bears?

Overtime the pond slowly becomes more acidic due to the release of chemicals from a nearby factory. which of the organisms would most likely survive the change to their environment?

Some scientists argue that global warming is related to buming fossil fuels to produce energy o increased use of fertilizers loss of forested areas a of the above