What was the reason for the British attack at the Somme? (Example of 4 mark question)

The Battle of the Somme in 1916 is known as one of the most bloody battles that Britain has participated in, with estimates that over one million soldiers had died by its conclusion in November.The primary reason for the british attack was to relieve Verdun; a French city under attack by the German army. By July, around 700,000 French casualties had been sustained by the French. This worried allied commanders like Haig who believed a counter-attack was neccessary in order to relieve pressure. The battlefield was chosen as it was experiencing relatively little fighting in early 1916, but was close enough to Verdun to force the germans to stop their counter-attack.The belief also held that a massive artillery bombardment would kill most of the Germans in their trenches and so the conditions of a river basin, like the Somme, were perfect for the bombarment Haig thought would be crucial to victory.