21. A uniform and constant magnetic field B is directed into the page within a rectangular region as shown in the figure. A wire in the shape of a quarter-circle is rotated at a constant angular velocity about an axis A. Which of the following graph represents the emf $\varepsilon$ induced in the wire as a function of time?

![img-11.jpeg](img-11.jpeg)
(A)$\left( \begin{array}{lllll} \varepsilon & & & \\ & & \text{ B} & \\ & & & \end{array} \right) \left( \begin{array}{llll} \varepsilon & & & \\ & & \text{ C} & \\ & & & \end{array} \right)$
(C)
(D)

(E)None of the above

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