Check answers?

what is the case of the underlined pronouns in the following sentence? is the umbrella yours or mine? (yours or mine is underlined)
objective
possessive < --
nominative

which of the following pronouns completes the sentence correctly? each of the boys began to clean desk.
his < --
him
their

1. Is the umbrella yours or mine? 
The pronoun yours is a possessive pronoun because it refers to somebody owning something - in this case, an umbrella.
2. Each of the boys began to clean his desk.
You don't have to use their here because we know that both persons are male - you would use their if the sentence said: Each of the children...