<s>[INST] context: Anti-larval measures of plasmodium (i) Larvicides : During the first half of the 20th century, anti-larval measures such as oiling the collections of standing water or dusting them with paris green effectively controlled malaria (but the measures were eclipsed at the end of World War II). With the increase in insecticide resistance, the older methods of mosquito control have now become promising. Some modern larvicides such as temephos which confer long effect with low toxicity are more widely used. However larviciding must be repeated at frequent intervals and for this reason it is a comparatively costly operation. (ii) Source reduction : Techniques to reduce mosquito breeding sites (often called source reduction) which include drainage or filling, deepening or flushing, management of water level, changing the salt content of water and intermittent irrigation are among the classical methods of malaria control to which attention is being paid again. Whenever practicable, measures for the improvement of the environment by the permanent reduction of sources should be instituted. {iii) Integrated control : In order to reduce too much dependance on residual insecticides, increasing emphasis is being put on "integrated" vector control methodology which includes bioenvironmental and personal protection measure .This approach is impoant because there is no single and simple method that would ensure control of transmission. By mid 1995 all malaria endemic countries in the region had adopted the revised malaria control strategy to reduce morbidity and moality and to reduce its area of distribution, paicularly of multidrug resistant malaria. The use of stratification approach by the majority of anti-malaria programmes in the Region has led to more cost-effective interventions. Vector resistance to insecticides has necessitated the use of more expensive pyrethroid, thereby limiting the coverage. Malaria control added impetus as Roll Back Malaria initiative was launched by WHO, UNICEF, UNDP and the World Bank in 1998. [/INST] question: Most efficient ani-larval measure to prevent urban malaria is | options: A: Clean drainage and sewage system B: Cover overhead tank C: Filling cesspools and ditches D: Cover pits | answer: B </s>