What is the minimum distance we should keep from the vehicle ahead inside a tunnel?

What is the minimum distance we should keep from the vehicle ahead inside a tunnel?
A)That which allows us to brake without colliding with the vehicle ahead.
B)At least 100 metres.Correct
C)At least 150 metres.
In tunnels or underpasses, when not intending to overtake, a safety distance from the preceding vehicle of at least 100 metres or a minimum time interval of four seconds must be maintained at all times.
For vehicles with a MAM exceeding 3,500 kg, the safety distance to be kept will be at least 150 metres or a minimum interval of 6 seconds.
Remember that overtaking is prohibited in tunnels, unless we have more than one lane in our direction.
The following sign reminds us of the general safety distance we must keep.