When to use 'What is' and 'What are'

To be sure when to use 'What is' and 'What are', look at the object. If it is singular, then use 'What is'. On the other hand, use 'What are' if it is plural.

The following examples should serve to explain when I mean:
a) What is the thing that you are holding? [The object 'the thing' is singular.]
b) What are those vegetables you are selling? [The object 'those vegetables' is plural.]