Subject & object questions

Most questions are object questions. They ask about an object.
Where do you live?
Who did you see? 

And there are also subject questions. Questions we ask to find out about the subject. These questions are asked using 'who', 'what' and 'which'.  
Who plays football in this class?
What happened?
Which machine did Marconi invent? 

You can take a look at the following chart of question forms:

Form	Examples
Object Questions wh- + auxiliary + subject + main verb	Where do you live? What will you do? When is she coming?
Subject Questions wh- (=subject) + auxiliary + main verb   Notice that this structure is like affirmative sentence structure without the question word.	Who loves you? Which car will arrive first? What type of food costs less?

Now, try this Hot Potatoes exercise and choose the correct question.