Verified answer

-K'nex

-tinker toys (if you have an awesome grandma like me who still has the old school wooden ones!)

-could get creative with legos

-toothpicks/popsicle sticks and wood glue

This question is bogus. Watson & Crick indeed proposed the answer given in A), but they DID NOT directly propose the semi-conservative model of replication. From their initial insight regarding the implications of complementary strands, a general consensus developed among ALL major investigators in the field that replication could be via one of three possible mechanisms: conservative, semi-conservative or dispersive. Watson & Crick's own work did not distinguish between those possibilities, and they made no specific proposal regarding which of the three mechanisms was most likely. The question remained open until Meselson & Stahl came up with an elegant way to address the issue. The closest here to a correct answer (i.e. the one your historically-challenged teacher is looking for) is A), but the whole question should be tossed out because it's initial premise is grossly misleading and outright wrong. Other posted answer is incorrect--Meselson and Stahl's experiments were conducted 5 years AFTER Watson & Crick's initial report, and were conducted in order to distinguish between the 3 possible mechanisms of replication--there was no way W&C could have used data from an experiment that was still 5 years in the future. From now on, when you think of semi-conservative replication, you need to think of Meselson & Stahl, NOT Watson & Crick.