o convex sets.</label></li>
                <li><label><input type="radio" name="q0" value="3"> d) Taking the intersection of two convex sets.</label></li>
            </ul>
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        <div class="question">
            <p>Let \( f : \mathbb{R}^d \to \mathbb{R} \) be twice continuously differentiable. Which of the following conditions is sufficient for \( f \) to be convex?</p>
            <ul class="options">
                <li><label><i