Write the correct relative pronoun (who, which, that, whose, where) to complete the following sentence:

NBC News special correspondent and former "Today" show host Tom Brokaw has multiple myeloma, ___ is a cancer of plasma cells.

Write the correct relative pronoun (who, which, that, whose, where) to complete the following sentence:

Spinal disks are pulpy masses ___ cushion pairs of vertebrae.

Choose the modal auxiliary which best completes the sentence:

This council estate ___ been built so close to the nuclear power station. It was a bad mistake.