阅读下面短文，按照句子结构的语法性和上下文连贯的要求，在空格处填入一个（最多两个）适当的词或使用括号中词语的适当形式填空。
On November 5th, 1872, the ship Mary Celeste left New York ____ Italy. The captain (船长), his wife and daughter and seven sailors(海员) were on this ship. But something terrible happened. The captain of another ship ____ (find) Mary Celeste floating (漂浮) in the middle of the sea on December 4th, 1872. ____, nobody was on the ship.
____ were the captain, his family and the sailors? Why did they leave the ship? Mary Celeste was not ____ (break). Everything was good. It seemed that the captain, his family and the sailors had ____ (leave) the ship in a hurry. We could tell all these because there was half-eaten food on the table. The sailors also left all ____ (they) clothing behind. The lifeboat (救生艇) was still on the ship. The captain's diary was found on the ship and the last passage ____ (write) on November 24th.
What happened ____November 24th and December 4th. How and when did the people leave the ship? So far, nobody can answer all these ____ (question). It is still a mystery.
＋2x－4y＝0的圆心，则a的值为