All bridges built from 1950 to 1960 are in serious need of rehabilitat

The question asks for what MUST be true based on the information given. For formal logic questions like this one, it is very helpful, and maybe even necessary, to diagram the relationships between and among the different elements. The diagram for this stimulus is as follows:

Faulty <--some--> built 1950-60 --all--> in serious need of rehab --some are not --> suspension bridges <--none--> faulty

Now let's see which of the answer choices MUST BE TRUE according to our diagram.

A) Some suspension bridges are not in serious need of rehabilitation.

- Nope, not necessarily. We know that some bridges in serious need of rehab are not suspension bridges, but this does not necessarily mean that some suspension bridges are not in serious need of rehab. To simplify further, suppose there are 10 bridges in serious need of rehab. 3 of them are not suspension bridges (satisfies the requirement that some are not suspension bridges). What about the other 7? Could 1 or more or even all of them be suspension bridges? Absolutely. Now what if there are only 7 suspension bridges, and they all happen to be the 7 that need rehab? Then we can say that all suspension bridges are in serious need of rehab. Hence, A does not necessarily have to be true.

(B) Some suspension bridges are in serious need of rehabilitation.

- Again, this does not necessarily have to be true. In fact, there is nothing in the stimulus that supports this. This could be true, it could be false.

(C) Some bridges that were built according to faulty engineering design are in serious need of rehabilitation.

- This is the correct answer. Looking at our diagram, we see that some bridges that were built according to faulty design where built between 1950-1960, and all bridges built between 1950 and 1960 are in serious need of rehab. Hence, it follows that some bridges built according to faulty design are in serious need of rehab.

One can argue that the stimulus only states that some bridges built between 1950-1960 are in serious need of rehab, and not the other way around. But consider the following scenario: 10 bridges were built during 1950-1960. 3 of them were built according to faulty design (satisfying the requirement in the stimulus). From this, we can clearly see that at least 3 bridges built according to faulty design were built during 1950-1960. Hence, some bridges built according to faulty design were built during 1950-60.

(D) Some bridges built from 1950 to 1960 are not in serious need of rehabilitation.

- This directly contradicts the information in the stimulus and is therefore false.

(E) Some bridges that were built according to faulty engineering design are not bridges other than suspension bridges.

- Nothing in the stimulus supports this. Could be true, could be false. We are looking for the answer that must be true.