Вопрос по английскому языку:

Put the verb in the correct form. Put the answers only in full forms with one space between them!
The Benton Horde, a priceless collection of ancient Roman silver, 14. _______________ (disappear) from its display cabinet in the British Museum. At today‘s emergency press conference, the Museum director John Clive said, that according to his theory, the treasures 15. ____________________ (probably/steal) by a professional gang on behalf of a specific collector. The matter is that the stolen silver 16. _______________ (be) very well-known and it 17. ____________________ (never/sell) on the open market.
Ten museums 18. _______________ (rob) in the last six months and the number 19. ________ (rise) steadily over the last few years. The Benton Horde 20. _______________ (consider) one of Britain‘s most important collections of Roman silver. It 21. _______________ (lie) undiscovered for almost sixteen centuries beneath a Sussex field until it 22. _______________ (find) incidentally in 1926.