题目

    A new study has found no evidence that sunscreen, commonly used to reduce the risk of skin cancer, actually increase the risk.     Researchers from the University of Iowa based their findings on a review of 18 earlier studies that looked at the association between sunscreen use and melanoma (黑素瘤). They said that they found flaws in studies that had reported associations between sunscreen use and higher risk of melanoma  Most health experts believe that by protecting the skin from the harmful effects of the sun, sunscreen helps prevent skin cancer, which is increasing in incide如图所示，一物体在水平恒力的作用下沿光滑水平面做曲线运动，当物体从M点运动到N点时，其速度方向恰好改变了90°，则物体从M点到N点的运动过程中，物体的速度将（ ）A．不断增大 B．不断减小C．先增大后减小 D．先减小后增大