如果直线ax+2y+1=0与直线x+y﹣2=0互相垂直,那么a的值等于( )
He was just falling asleep _____ there was a loud knock on the door.
图中五角星旋转360°图形发生变化会吗?____(填”会“或者”不会“)
请根据以上叙述,总结出引起少年儿童肥胖的主要原因:____。