Based on the information present in the given passage, you need to write a correct answer to the given question. A correct answer is one that correctly and completely answers the question. Do not write answers that address the question partially or incorrectly. The correct answer must be contained in the given paragraph.
Q: Paragraph- Sent 1: The opening shot of the movie shows Kunti praying for Lord Krishna's protection for the Pandavas .
Sent 2: Lord Krishna consoles Kunti and promises to ever protect the Pandavas and guide them through troubles and problems that may occur in life .
Sent 3: The sons of Pandu and Dhritarashtra progeny break into an argument .
Sent 4: When Duryodhana insults the Pandavas as `` dependents '' , Bheema counters by saying that , the Kauravas are the progeny of a widow .
Sent 5: Duryodhana asks Veda Vyasa for an explanation .
Sent 6: He is then told that , since his mother , Gandhari had an astrological defect , she is first married of to a goat and then married to his father .
Sent 7: Duryodhana gains animosity towards the kingdom of Gandhara where the king , the father of his mother Gandhari , rules .
Sent 8: He attacks Gandhara and lays waste of the whole kingdom .
Sent 9: He them imprisons the royal family in his prison .
Sent 10: He gives them only one rice grain per prisoner .
Sent 11: The king of Gandhara then stops everyone from grabbing the little food that is provided .
Sent 12: He says that instead of everyone dying , they could keep at least one of their princes alive .
Sent 13: He chooses Sakuni to be alive .
Sent 14: Sakuni takes an oath that he will do everything he can to destroy the entire Kaurava clan .
Sent 15: He makes magic dice from his father's spinal cord .
Sent 16: The magic dice show exactly the number that he would want .
Sent 17: Duryodhana takes pity on the lone prisoner , Sakuni after the rest of the Gandhara royal family dies in prison out of starvation .
Sent 18: Sakuni joins the evil of coterie of Duryodhana , Karna and Dushyasana . 
Question: Which prince do the king of Gandhara choose to keep and not die?.
A: 
He takes an oath.