用所给动词的适当形式填空：The steamboat____ (sail) up the river.

可以有几种填法：
The steamboat is sailing up the river. 船正顺河而上。
The steamboat has sailed up the river. 已经... ...
The steamboat sailed up the river. (描述过去发生的动作）
The steamboat was sailing up the river. (描述过去正在发生的动作）
The steamboat will sail up the river. 即将......

sailed
is sailing

sails语境上用一般现在时

sailed

sails