Sorry Stable, but the answer is 'Incorrect'.
You need to remember antenna geometry. An aircraft at low altitude will not be detected by another at high altitude, because the transponder antenna systems are not intended to (and do not) offer extensive coverage in terms of elevation, or, traffic at significantly different altitudes, close in. Witness flying over London at FL200+ and getting no TAs at all on light aircraft around the zone.
TCAS will NEVER give an RA on a Mode A target, and will sometimes give a TA provided that the target aircraft is at a relatively similar level to the TCAS-equipped aircraft, bearing in mind its range and range by range rate.
A recent incident does however point out why we should be cautious of TAs on non-mode C targets. Basically, as I understand it, a light twin departed Cranfield for Amsterdam, failed to establish two-way contact with London Info, and so climbed to FL70 on direct track to SPL. Traffic around Stansted got TAs on the aircraft, and there is a danger that pilots in this situation think 'it's at low level'. 'It' may not be.