If the relative clause is of such a nature that it could be introduced by and he, and she, and it, and they, etc., the relative who (for persons) and which (for animals or things) should be used in preference to the relative that.
“Man that is born of a woman is of few days, and full of trouble.” The language of the Bible and of Shakespeare must stand, although the forms of expression differ greatly from those employed at the present day. According to modern standards, that should be who.
“The earth is enveloped by an ocean of air that is a compound of oxygen and nitrogen!” Change that to which.
The relative that should be used in preference to who or which:
(1) When the antecedent names both persons and things;
(2) When it would prevent ambiguity;
(3) After the words same, very, all;
(4) After the interrogative pronoun who;
(5) After adjectives expressing quality in the highest degree.
“The wisest men who ever lived made mistakes.” Use that. See (5).
“He lived near a stagnant pool which was a nuisance.” Use that. See (2).
“All who knew him loved him.” Say that. See (3).
“Who who saw him did not pity him.” See (4).
“He spake of the men and things which he had seen.” See (1).