­Historian: In a territorial dispute in the mid-1700's, Ulmias navy

Blockade of a port means ships are not allowed to enter or leave. We are given that merchant ships were not allowed to enter or leave and it is believed that this forced Crinica to resolve the dispute (whatever it was) in favour of Ulmia.  

But the historian claims that only 1 in 6 merchant ships were captured. So the blockade had only a 16% impact (and hence may not have been the reason that forced Crinica to give in)

We need to weaken the historian's conclusion (that blackade was not the reason) so we have to show that the blockade could have put pressure on Crinica to resolve the issue in Ulmia's favour. 

A. Crinica had no means of preventing Ulmia from blockading its ports.

Whether Crinica had means or did not have, doesn't matter. We know that 1 in 6 merchant ships were captured and hence the impact of the blockade was limited. No reason to say that the. histroian may be wrong. 

B. Prior to the blockade, the vast majority of ships entering or leaving Crinica's ports were owned by residents of Crinica itself.

We are talking about merchant ships. Whether those merchants were Crinica's residents or others, doesn't matter. Who the ships belonged to, we don't care. 

C. Only a relatively small proportion of Crinica’s overseas trade was with Ulmia prior to the blockade.

What proportion of Crinica's trade was with Ulmia before the blockade, doesn't matter. In fact, we don't know and don't care how much was it during the blockade too. We don't know what the dispute was. All we know is that Ulmia was blocking Crinica's merchant ships. Where were those ships headed, we don't care. 

D. The volume of Ulmia’s overseas trade with countries other than Crinica was not significantly affected during the period of the blockade.

This doesn't weaken the historian's conclusion. In fact, it supports it in a way. If trade was not significantly affected, then Crinica would not have been under pressure to give in due to the blockade. Hence it makes sense that the blockade would not have been the reason for the outcome.

E. The number of ships captured each month increased sharply in the latter stages of the blockade as Ulmia greatly expanded its blockade force.

If this is true, then the historian's logic doesn't make sense. Over the course of the blockade, on average only 1 in 6 ships were captured but what if the frequency of capture increased over time. 

e.g. in the 1st year, only 1% ships were captured. 
In the second year, only 4% were captured. 
In the third year, only 10% were captured.
In the last year, the percentage kept increasing till EVERY ship was captured as the 4th year came to an end. This put pressure on Crinica and hence Crinica gave in. 
This could have given us an overall 16% ships captured over the 4 years, a number that doesn't seem to be huge but the pressure was much lower in the initial years and increased substantially in the later stages. 

Answer (E)
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