题目

阅读下面短文，把A-D四个句子填入文中空缺处（1-4），使短文内容完整正确。 Summer holidays are coming soon．Are you going to travel? If you plan to take a plane，you will have a healthy trip by following these suggestions（建议）．1．Bring some needed medicines in your carry-on bag． 2．Don't leave your seat during the plane's take offsand landings． 3．To prevent blood clots（血流不畅）： ★1 _______ ★Do leg - stretching（伸）exercises in your seat． ★Wear comfortable clothes． 4．If you sometimes get airsick： ★2 _______ ★Keep yo下列说法正确的是（　　）A．只要有光照射在金属表面上，就一定能发生光电效应B．一群处于n=4能级的氢原子向低能级跃迁时能辐射出六种不同频率的光子C． 23892U→ 23490Th+ 42He是a衰变方程D． 23592U+ 10n→ 14054Xe+9438Sr+d10n，式中d=3