题目

       The truth  46  trees are vital to our life is not a secret. They provide us with food, wood and most 47  (important), oxygen. Now there is one more thing we can add to this list—blocking out harmful bacteria from water.111]     The discovery 48  (make) by a team consisting 49  scientists from the Massachusetts Institute of Technology (MIT) and high school students 50  were seeking a natural water filter—one that would help communities in developing countries that do not have access to modern water filter systems.     The 51  (research), led by Professor R一列简谐横波沿x轴传播，周期为T，t=0时刻的波形如图所示。此时平衡位置位于x=3 m处的质点正在向上运动，若a、b两质点平衡位置的坐标分别为xa=2.5 m，xb=5.5 m，则 [     ]A.当a质点处在波峰时，b质点恰在波谷 B.t=T/4时，a质点正在向y轴负方向运动 C.t=3T/4时，b质点正在向y轴负方向运动 D.在某一时刻，a、b两质点的位移和速度可能相同