Which of the following pairs of substances form a buffer system for human blood? A.
HCl and Cl-
B.
NH3 and NH2-
C.
H3C6H5O7 and HC6H5O72-
D.
H2CO3 and HCO3-

d. Because those chemicals are easily made when CO2 reacts with water, forming H2CO3 (via carbonic anhydrase

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.

the answer is 0.42 is that is an option. this question provides no info