The overall change in welfare = Change in Consumer Surplus + Change in Producer Surplus + Change in Government Revenue = (-A-B-C-D) + A + C = -B-D.
厚生における全体にわたる変化 = 消費者余剰での変化 + 生産者余剰での変化 + 政府税収での変化 = ( - A - B - C - D ) + A + C = - B - D 。