Their ________ grew more profound as time went on.

Hint:

Suffix is a letter or group of letters added at the end of a word to make a new word or to change the word grammatically.
If a word ends in ‘–s’, ‘–sh’, ‘–ch’, ‘–x’, or ‘–z’, you add ‘–es’. For almost all other words that do not end with ‘-s’, ‘-sh’, ‘-ch’, ‘-x’ or ‘-z’, add ‘–s’ to make it plural.
The suffix ‘-ed’ is added to a word to make it in the past tense or the past participle.
The suffixes ‘-tion’, ‘-sion’, ‘-ment’, ‘-ness’, etc. are used to form a noun of the given word.
The suffixes ‘-able’, ‘-ful’, ‘-ive’, ‘-less’, etc. are used to form an adjective of the given word.