Read the sentences.

Glazing pottery can be tricky. The stunning results make the effort worthwhile.

What should be included to best combine the sentences?

O

, and

, but

when

The word which best combines the two sentences is but.

But is a conjunction. It is used as a linking word to connect contrasting ideas.

Here, we are told that pottery-glazing is a difficult task, which is a bit negative. However, we are also told that the results of this task is a beautiful object, which is enough to justify the hard work. This is positive.

To contrast the negative and the positive informations, you should use but.

"Glazing pottery can be tricky, but he stunning results make the effort worthwhile."