Catalysts affect the chemical reactions that take place within the body. what function do catalysts perform in chemical reactions?
a. catalysts slow down the rate of chemical reactions.
b. catalysts are always formed by chemical reactions.
c. catalysts are used up during a chemical reaction.
d. catalysts lower the activation energy of a chemical reaction.

c. heat could easily travel through the molecules in the air and escape/enter the flask. removing the air slows down the heat transfer.  

nuclear fission produces elements that are heavier than helium.

the elements that are used in nuclear fission and their products are much heavier than helium.

answer: atomic mass was originally used to arrange elements on the periodic table, but is no longer used to organize the modern version.

explanation:

originally atomic mass was used to arrange elements on the periodic table from left to right in a row. each column then shows similar properties of elements of a group.

in modern periodic table, the atoms are arranged in increasing atomic number. as atomic number represents the number of protons in an atom which is unique for every atom.