From the passage we know that the security measures taken by Metro are ______.
A: in the wake of the Madrid bomb blasts
B: not visible to the passengers or passers-by
C: extensive in its coverage of public areas
D: various ranging from the use of dogs to the access to FBI
A: in the wake of the Madrid bomb blasts
B: not visible to the passengers or passers-by
C: extensive in its coverage of public areas
D: various ranging from the use of dogs to the access to FBI