9. Pierre said, "Just one of us is telling the truth". Qadr said, "What Pierre says is not true".
Ratna said, "What Qadr says is not true".
Sven said, "What Ratna says is not true".
Tanya said, "What Sven says is not true".
How many of them were telling the truth?
A 0
B 1
C 2
D 3
E 4
Solution: $\mathbf{C}$
If Pierre is telling the truth, everyone else is not telling the truth. But, also in this case, what Qadr said is not true, and hence Ratna is telling the truth. So we have a contradiction.

So we deduce that Pierre is not telling the truth. So Qadr is telling the truth. Hence Ratna is not telling the truth. So Sven is also telling the truth, and hence Tanya is not telling the truth. So Qadr and Sven are telling the truth and the other three are not telling the truth.