that, those

Hi.

My question is regarding the usage of that/this and those/these.

For instance, I hear the phrase 'things like that' being used even when 'that' seems to refer to many things!

I thought of the following, and was wondering how I should proceed...

a) Problems such as THAT discussed by the Professor were...(One problem has been previously discussed)

b) Problems such as THOSE discussed by the Professor were..(More that one problem has been previously discussed)

c) Problems such as THIS...(one previous problem)

d) Problems such as THOSE/THESE (more than one problem has been discussed previously)

Am I on the right lines, or do I need serious help?!!

Thanks for any help that you can provide...