4. Chinese President has officially opened the world's longest sea crossing bridge that __ in 2016 but couldn't due to safety issues at that time. A)must have opened B)can open C)had better open D) should have opened E) ought to open

The correct answer is D) should have opened. This option is the most appropriate because it implies that the Chinese President was expected to open the bridge in 2016, but due to safety issues at that time, it didn't happen. The other options do not convey the same sense of expectation and necessity.