Which of the following is an element?

a. o2
b. cacl2
c. hpo4
d. co2

(also i'm freshman in high school, idk why it's saying middle school)

Gas will most likely  fill the container 

Ithink it’s 98 because jajajjajansmsmjs

For me a person with a phd and who knows what there talking about.i know the chances of a radiation leak is low but if it does the area within a 96.3 mile radius would be unusable because of contamination

Ok what do you need with