State the status of the contract of sale (valid, void, enforceable, unenforceable) of a piece of land if:

a. Authority of the agent to sell a piece of land is in writing

b. Authority of the agent to sell a piece of land is not in writing

c. Authority of the agent to sell a piece of land is in a private instrument, and the contract of sale was

i. Entered orally

ii. In a private instrument

iii. In a public instrument

d. Authority of the agent to sell a piece of land is in a public instrument, and the contract of sale was

i. Entered orally

ii. In a private instrument

iii. In a public instrument