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{T}: Add {C}.
{3}, {T}: Target creature with power 3 or less can't be blocked this turn.
If the target creature's power is greater than 3 as Access Tunnel's second ability tries to resolve, the ability doesn't resolve. However, if instead the creature's power is raised above 3 after the ability resolves, it still can't be blocked that turn.
Once a creature with power 3 or less has been blocked, Access Tunnel's second ability won't cause it to become unblocked.
