A 25-year-old woman presents to the emergency department with polyuria, polydipsia, abdominal pain and vomiting. She has type 1 diabetes. Vitals: HR 112, BP 100/60 mm Hg, RR 26, Kussmaul respirations. Glucose is 25 mmol/L, pH 7.15, serum bicarbonate 10 mmol/L, serum potassium 5.2 mmol/L. Which is the most appropriate initial intravenous fluid therapy?