[Used Car Commission] William Promised to Sell Caileen's Car for Her

Multiple Choice

[Used Car Commission] William promised to sell Caileen's car for her, but he wanted a commission of 10%. Caileen signed an instrument promising to pay William a 10% commission if he sold her car. William assigned the agreement to Mehdi. Caileen's car was sold and the buyer paid Caileen. A dispute ensued between Caileen and William regarding whether William found the buyer or the buyer found Caileen. When Mehdi asked Caileen for payment on the instrument, Caileen refused. William, Caileen, and Mehdi settled their dispute without going to court and Caileen wrote Mehdi a check for $3,000. Mehdi endorsed the check on the back by signing his name planning to take it to the bank the next day. Unfortunately, Mehdi lost the check which was found by Caileen and cashed by the local bank. Caileen then left town.
-Which of the following is true regarding the instrument signed by Caileen promising to pay William a 10% commission if he sold her car?

A) The instrument is negotiable.
B) The instrument is not negotiable only for the reason that it is based on a condition.
C) The instrument is not negotiable only for the reason that Caileen is not a merchant.
D) The instrument is not negotiable only for the reason that it is not for a sum certain.
E) The instrument is not negotiable because it is based on a condition and also because it is not for a sum certain.