a,c 都不是正确答案!
给你一个正确的：d.will children be...
Children will be no longer the only people who have time to play.
----No longer will children be the only people.
当含有 no/not.的时间或方式状语短语提到句首时,引起句子的部分倒装.
常见的：in no time,in no case,by no means,on no account,not a single,in no circumstances,in no way,etc.
e.g He didn't make a single mistake in the test.
--- Not a single mistake did he make in the test.