In spanish what's the difference between direct object pronouns and indirect object pronouns

Direct object pronouns are used to replace direct object nouns, but indirect object pronouns are used to replace nouns that act as indirect objects. Also, indirect object pronouns answer to questions such as: to whom? or for whom? about the verb, but direct object pronouns answer to questions such as: whom? or what? about the verb

These objects represent mayan numerals. just like roman numerals, they are combined to symbolize a numeral and this enabled astronomers, scholars and architects to compute various equations yet it was also used by farmers and traders in their day to day transactions.

Ithink its b) built underground reservoirs