题目

完型填空 阅读下面短文，掌握其大意，然后从每题所给的四个选项（A、B、C和D）中，选出最佳选项  At present, almost everyone has a mobile phone, but have you ever thought that cell phones   1   be bad for your health? A study shows that heavy mobile phone use may cause hearing loss.  This study shows that people who use cell phones for about an hour a day have a   2   time hearing some similar sounds, especially in their right ears. It is getting harder for   3  to tell the difference between the sounds of “s” and “f”, and “t” and 正四棱柱ABCD-A1B1C1D1中，底面边长为a，侧棱AA1长为ka（k＞0），E为侧棱BB1的中点，记以AD1为棱，EAD1，A1AD1为面的二面角大小为θ．（1）是否存在k值，使直线AE⊥平面A1D1E，若存在，求出k值；若不存在，说明理由；（2）试比较tanθ与的大小．