Trong không gian với hệ toạ độ Oxyz, cho mặt phẳng (P):6x-2y+z-35=0 và điểm A(-1;3;6). Gọi A′ là điểm đối xứng của A qua (P). Tính OA′.

A. 3 26

B.  5 3

C.  46

D.  186

Read the text and choose the correct answer for each question below: 

Ha Long Bay

Ha Long Bay is a group of offshore islands that is the best example of marine invaded limestone region in the world.  The Bay holds over 1,600 islands and islets. There are caves and grottoes, with stalactites and stalagmites. Its limestone pillars are a unique natural feature of great scenic beauty and biological interest. The great extent and the richness of its forms set it apart from many other sites.

Ha Long Bay is an extremely popular destination both for foreigners and Vietnamese: already in the early 1990s it saw over 1 million visitors a year. It is unlikely that the number has decreased over the years - so there will always be about 3,000 other people in the Bay at the same time as you!

The natural beauty of the Bay lay hidden today under a very common fog. As I had already seen the similar karst landscape in China a couple of years before, I certainly said that I was blown away by the beauty of Ha Long Bay. We went onto one of the islands to visit the Surprising Cave - an indeed surprisingly big and beautiful cave with three hollow chambers. We saw some monkeys here too, just outside the cave exit.

Part of the tour was half an hour of kayaking - enough to paddle a full circle around the main area. The views from the kayak I found much more impressive than from the larger boat: you're so tiny then and the peaks rise sharply in front of you. I had a better look at the water too, and cannot say that I saw pollution by plastic or other junk floating around.  There is a thin layer of oil on the water in some parts though.

And then it's time to get back in the bus to Ha Noi, another 3.5 hours. The tour was carried out well with good seafood for lunch and I had a satisfying day.

Question: All of the following statements are true about Ha Long Bay EXCEPT that ______.

A. the writer thought its beauty surpassed many other sites

B. the writer didn’t think the number of tourists had decreased over the years

C. the writer saw some monkeys in the chambers of the Surprising Cave

D. it took three and a half hours to travel from Ha Noi to Ha Long by bus