How can you say that water contains hydrogen/oxygen?

Write the chemical symbols for three different atoms or atomic cations with 27 electrons. asap!

The diagram shows the structures of horse and cat forelimbs. what does the diagram suggest about the evolutionary relationship between these two mammals? a. they have homologous structures, indicating a common ancestor. b. they have analogous structures, indicating a common ancestor. c. they have homologous structures, indicating that they do not have a common ancestor. d. they have analogous structures, indicating that they do not have a common ancestor.

Can someone me with 3 and 4 plz. this is for masteries test.