अभिव्यक्ति 'six years ago' में रेखांकित शब्द क्या है?

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'.

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