A 24 year old man with type 1 diabetes is brought to the emergency department with abdominal pain, vomiting and drowsiness. He has missed several insulin injections. On examination he is dehydrated, tachypnoeic with Kussmaul respirations and his capillary blood glucose is 28 mmol/L. Urine dipstick is strongly positive for ketones. Which of the following is the most appropriate initial intravenous fluid to start?