A. The phrase a number of is always treated as plural.
A. Be careful when the number (singular or plural) of the predicate noun and the subject differ.
The multiple design flaws are the most urgent issue.
The most urgent issue is the multiple design flaws.
His best feature is his eyes.
His eyes are his best feature.
The only problem with the car is its broken windows.
Most important is the children.
The children are most important.
The only deductible item is my business expenses.
My business expenses are the only deductible item.