单选题 But the insidious nature of the virus, its capacity to spread and kill remained, and its lingering mysteries — how it's transmitted, why it's more virulent in some people than others, how it's best treated — have public-health experts discernibly worried and unapologetic for erring on the side of caution.
【正确答案】 D
【答案解析】解析：concise简明的，简洁的；equivalent相等的，相当的；morbid由疾病引起的；pestilent致命的。句意是，病毒的潜在本质、它保有的传播和杀死的能力及其神秘性，例如其如何传播，为什么对某些人更有致命性，以及如何治疗等，所有这些令公共安全专家甚是担心。所以正确答案为D。