用所给词的适当形式填空  ：
(1)Who can jump the ____ (far) in your class, Sandy, Lucy or Kate?
(2)Chen Kaige is one of the famous ____ (direct) in China.
(3)The boy's ____  (die) is a great disaster to his parents.
(4)There's always a risk of getting killed, so the family has to move away to another city for ____ (safe).
(5)Shortly after, the ____ (France) director told me that he had devoted his lifetime to the industry.
(6)Chinese people have found out the____  (true) through many years' struggle.

填入下面这段文字横线处的语句，最恰当的一句是（    ）

    俗话说“病从口入”，这当然是指不能乱吃东西，同时也是提醒大众要注意口腔卫生。            。随着人们生活水平和卫生意识的提高，洁牙已经是一件日常的理所应当的事。

已知a^m=3，aⁿ=21，求a^m+n的值

位于美国富兰克林的小依科庞德湖，湖水的pH为4.2，是世界上____性最强的湖。该湖水是否适宜一般作物的灌溉____。（“是”或“否”）