X. Open the brackets and put the verbs in the required

X. Open the brackets and put the verbs in the required passive form. Mind the tense and preposition.
1. The application of these mining methods often (to refer to).
2. The process of extracting lead from this ore (to objectd to) at the recent conference.
3. The statistical theory (develop) quite recently.
4. Objects with negative stability usually (to call) unstable.
5. These classifications already (to use).
6. Cooling often (to provide) by the circulation of water.
7. Some pressing problems (to discuss) at the symposium next week.
8. The new discovery (speak about) now.
9. The scientists understood that a more careful approach (to need) to realize their idea in future.
10. The explorers (to see) not far from the camp of the expedition yesterday.