I am pretty sure that that the answer is A.
The extension of the war beyond the end of 1914 resulted from <u>B</u>. Belgian resistance to the Schlieffen Plan.
<h3>What was the Schlieffen Plan?</h3>
The Schlieffen Plan was a strategic German military maneuver to sweep France and Belgium to frustrate Russia from coordinating an allied attack. Described as a strategy for victory, the plan was largely frustrated, thereby infuriating the Germans to device more plans to prosecute a decisive offence.
Thus, Belgian resistance to the Schlieffen Plan was responsible for extending the war beyond the end of 1914.
Learn more about the Schlieffen Plan at brainly.com/question/341707
A historian would consult a secondary source in order to:
option A. read entertaining stories about an event.