题目

A father in Wuhan, nicknamed Eagle Dad for his controversial parenting style, has dismissed critics' claims that he pushes his 4-year-old son too hard. Police officer Wang Shaoyan came under the spotlight this month after his child took part in an 18-kilometer race in the Hubei provincial capital. Some parents attacked his methods of extreme physical training, while others reacted strongly to his admission that he regularly exposes the youngster to cold showers. I just want my son to reach his full potential, Wang said, adding that 4-year-old son not only runs 3 km every day he also tak已知两个单位向量与的夹角为135°，则的充要条件是（ ）A．B．C．D．