In this task, you are given a question and a context passage. You have to answer the question based on the given passage.

What is the most common way to transmit email?, Context: Many mailbox providers are also access providers, while others are not (e.g., Yahoo! Mail, Outlook.com, Gmail, AOL Mail, Po box). The definition given in RFC 6650 covers email hosting services, as well as the relevant department of companies, universities, organizations, groups, and individuals that manage their mail servers themselves. The task is typically accomplished by implementing Simple Mail Transfer Protocol (SMTP) and possibly providing access to messages through Internet Message Access Protocol (IMAP), the Post Office Protocol, Webmail, or a proprietary protocol.
(SMTP)

Which category contains a 2 year old tree?, Context: To accurately map the Amazon's biomass and subsequent carbon related emissions, the classification of tree growth stages within different parts of the forest is crucial. In 2006 Tatiana Kuplich organized the trees of the Amazon into four categories: (1) mature forest, (2) regenerating forest [less than three years], (3) regenerating forest [between three and five years of regrowth], and (4) regenerating forest [eleven to eighteen years of continued development]. The researcher used a combination of Synthetic aperture radar (SAR) and Thematic Mapper (TM) to accurately place the different portions of the Amazon into one of the four classifications.
2

Which organ does race usually refer to?, Context: The US racial or ethnic classification "black" refers to people with all possible kinds of skin pigmentation, from the darkest through to the very lightest skin colors, including albinos, if they are believed by others to have West African ancestry (in any discernible percentage), or to exhibit cultural traits associated with being "African American". As a result, in the United States the term "black people" is not an indicator of skin color or ethnic origin but is instead a socially based racial classification related to being African American, with a family history associated with institutionalized slavery. Relatively dark-skinned people can be classified as white if they fulfill other social criteria of "whiteness", and relatively light-skinned people can be classified as black if they fulfill the social criteria for "blackness" in a particular setting.
skin