Researchers are using computer images to help surgeons plan difficult operations and to develop programs that will work for doctors and nurses in the same way that flight simulators do for pilots, letting medical personnel practice their techniques and test their reflexes before they ever see a patient.
想问一下，对于正确选项A，我没有理解letting后面的句式，这是个", ing"表示结果吗，然后是一个 let sb do的结构吗？并且最后的before they ever...是一个时间状语从句吗？像这种从句也可以加在没有主句的后面吗？随便一个结构后都可以加一个从句吗？
Read the statement and the instructions that follow it, and then make any notes that will help you plan your response. Begin typing your response in the box at the bottom of the screen.
The following appeared in a memorandum from a regional supervisor of post office operations.
"During a two-week study of pestal operations, the Presto City post office handled about twice as many items as the Lento City post office, even though the cities are about the same size. Moreover, customer satisfaction appears to be higher in Presto City, since the study found fewer complaints regarding the Presto city post office. Therefore, the postmasters at these two offices should exchange assignments: the Presto Coty postmaster will solve the problems of inefficiency and customer dissatisfaction at the Lento City office while the Lento City postmaster learns first-hand the superior methods of Presto City."
The Near Earth Asteroid Rendezvous (NEAR) spacecraft will orbit the asteroid Eros for a year, slowly moving closer to the surface of the object to make ever more precise measurements that scientists hope will enable them to understand how the solar system formed some four billion years ago.
Dressed as a man and using the name Robert Shurtleff, Deborah Sampson, the first woman to draw a soldier's pension, joined the Continental Army in 1782 at the age of 22, was injured three times, and was discharged in 1783 because she had become too ill to serve.