Hi younghoon27!
Glad to hear you ultimately chose the right answer! But I'd be happy to speak to why answer choice (B) is better than (A).
To understand why, try abstracting / concisely rewording what the stimulus is saying. This can be broken down into a handful of pieces:
(1) Precautionary measures are needed to assess costs before taking the next step
(2) Such precautionary measures have not yet been taken
(3) Therefore, the next step (of building in that location) should not be taken
Next, look for a similar abstraction in the answer choices. We can see this in answer (B):
(1) Precautionary measures are needed to assess costs before taking the next step ("Defective products can cost an appliance manufacturer millions of dollars because of product recalls and lawsuits.")
(2) Such precautionary measures have not yet been taken ("Yova Corporation’s new line of appliances has not yet been thoroughly tested for defects.")
(3) Therefore, the next step should not be taken ("Thus, Yova should not bring its new line to market at this time.")
This parallel structure is missing in answer choice (A). That answer choice states, "A new highway cannot be built in the lake district unless an environmental impact assessment is first carried out. An environmental impact assessment would cost more than the projected economic benefit of the highway. The proposal for a new highway in the lake district should therefore be rejected." This reasoning is simply stating that a cost-benefit analysis indicates that the proposal should be rejected; there is no cost-benefit analysis in the stimulus (it mentions the "cost" of needing to drain the marsh, but it doesn't base its conclusion purely on a cost-benefit analysis), which is why this answer is ultimately incorrect for a parallel the reasoning type of question. Answer choice (B), by contrast, much more neatly parallels the argumentation employed in the stimulus.