The missing boys were last seen ________ near the river.
正确答案： A
本题解析：
所填的词为主语补足语，因其后有明确的地点，补足语表示动作当时正在进行中，所以只能使用 playing 。
The house was quiet in the night, ________ the occasional coughing from my throat.
________ in thought, he almost ran into the car in front of him.
By the middle of the 21st century, the vast majority of the world ’s population will be living in cities _______ in the country.
The government called out policemen to suppress the riot.
The policeman came up to the lonely house with the door _______, _______ there for a while and then entered it.
_______ he goes, she will go, and she never cares _______ happens.
______ the new plan can be carried out will be discussed at the meeting tomorrow.
Please write a report ________ the above subjects are to be covered.