Which gas found in water do fish, clams, and snails use in order to breathe?

Which term refers to the total dollar value of the goods and services produced in a country in a year?

Which sentence best expresses the differing opinions of these two book reviews?

How does pregnancy begin? a) a placenta forms in the uterus b) a sperm reaches an egg in the fallopian tubec) the blastocyst differentiates into two cell types d) contractions begin in the uterine wallapex