A．They have difficulty identifying these ceils.
B．They aren’t sure how these cells work.
C．They’ve learned how to reproduce these cells.
D．They’ve found similar cells in other species.

A．They may crash because of nature disasters or human faults.
B．The world’s weather conditions are getting worse.
C．The airport safety inspections are getting worse.
D．Airplanes are often overloaded nowadays.