Verified answer

11 The cause of the damage was being proved to us by the contractors when the unexpected news came.

12 Mr. Thunder will have been introduced to us before the sale of Steelstock is agreed upon.

13 This camera must be declared (by you) to the Customs.

14 This idea could have been suggested to you. Thus a lot of trouble could have been avoided (by you).

15 This needn't be explained to anyone.