7 Mark for Review
\( 4 x^{2} \)
In a certain factory, assume that the number of workers is constant. The number of minutes \( N \) that it takes to make a single unit of a product and the number of units \( U \) of the product that are made per day satisfy the relationship \( U=\frac{k}{N} \), where \( k \) is a constant. Which of the following best describes the relationship between the rate of change, with respect to time \( t \), of \( U \) and the rate of change, with respect to time \( t \), of \( N \) ?
(A) \( \frac{d U}{d t}=\frac{k}{\left(\frac{d N}{d t}\right)} \)
(B) \( \frac{d U}{d t}=\frac{-k}{\left(\frac{d N}{d t}\right)} \)
(C) \( \frac{d U}{d t}=\frac{k}{N^{2}}\left(\frac{d N}{d t}\right) \)
(D) \( \frac{d U}{d t}=\frac{-k}{N^{2}}\left(\frac{d N}{d t}\right) \)