Q$12..$ A narrow$-$crested weir is a type of hydraulic structure commonly used for flow measurement and control in open channels such as rivers, streams, and irrigation canals. The term "narrow$-$crested" refers to the shape of the weir crest, which is relatively narrow compared to the width of the channel.

In the context of a narrow$-$crested weir, the width of the crest plays a significant role in determining its hydraulic characteristics. Specifically, the width of the crest is related to the height of water above the weir crest, known as the head. The ratio of the width of the crest to the head is an important factor in the discharge coefficient, which is used to calculate the flow rate over the weir.

Now, let's address the question directly. A narrow$-$crested weir is said to be such that the width of the crest is $\_\_\_\_\_\_\_\_\_$ half the height of water above the weir crest.

$($A$)$ Equal to

$($B$)$ Less than

$($C$)$ More than

$($D$)$ None of these