A wrote his brother B authorizing the latter to sell a parcel of land belonging to him located in Cagayan Valley. On the strength of such letter-authority, B sold the land. Is the sale valid?

• A. No, because the letter-authority must be in a public instrument.
• B. No, because the letter-authority must be participated in a notary public.
• C. Yes, because it is sufficient that the letter-authority be in writing.
• D. No, sale in invalid.