Answer:
4) long war of attrition. The Germans made it close to Paris and occupied Belgium, but other than that, it was mostly stagnate. Attempted pushes largely failed until around 1917-1918. The Germans attempted to break the allied line in 1917 but they were halted by a resupply due to Americans, and were broken.The Germans were pushed back more and more until they surrendered.
Step-by-step explanation:
1) The Germans did NOT have control of the seas. They had a U-Boat war similar to WW2, but they did not have control. While large, the German navy was outmatched by the British and French navies. Germany was blockaded, and slowly strangled.
2) Larger populations were not necessarily a factor, nor are they mentioned much in serious discussions of trench warfare's impact, but it could be possible.
3) Air power played some part in WW1, but it was greatly limited. Trenches were usually very barricaded and dug out. They were difficult to bomb out even in WW2, where the technology was way ahead.