In the sentence 'And that's just what I did', the word 'that' is used as a/an:

Identify the part of speech of the underlined word in the given sentence: However, not all soils can accumulate both mineral-associated organic matter.

Identify the part of speech of the word 'very' in the line: 'The very different lifetimes of particulate organic matter'.

The underlined word in the expression 'six years ago' is a/an: