Is the earth round?The little boy asked.(改为宾语从句)

题目

Is the earth round?The little boy asked.(改为宾语从句)
The little boy asked ______ the earth ______ round.
请问the earth 后用 is 还是 was?

答案

if,is 尽管主句the little boy asked是一般过去时,但the earth is round是客观事实,只能用一般现在时