Question10.5. Which of the following statements are true? A) Let $\mathbf{u}$ and $\mathbf{v}$ be any two vectors in $\mathbb{R}^{n}$. Then $\mathbf{u} \cdot \mathbf{v} \geq 0$. B) Let $\mathbf{u}$ and $\mathbf{v}$ be vectors in $\mathbb{R}^{n}$ such that $\mathbf{u} \cdot \mathbf{v}<0$. Then $\mathbf{u}=-c \mathbf{v}$, for some scalar $c>0$. C) Let $\mathbf{u}$ and $\mathbf{v}$ be vectors in $\mathbb{R}^{n}$ and let $\theta, 0 \leq \theta \leq \pi$ be the angle between them. If $\mathbf{u} \cdot \mathbf{v}<0$, then $\frac{\pi}{2}<\theta \leq \pi$. D) Let $\mathbf{u}$ and $\mathbf{v}$ be vectors in $\mathbb{R}^{n}$ such that $\mathbf{u} \cdot \mathbf{v}=0$. Then either $\mathbf{u}=\mathbf{0}$ or $\mathbf{v}=\mathbf{0}$.