What form of vitamin d is the major circulating form with a half-life of approximately 3 weeks?

The correct answer is calcifediol or calcidiol.
It is a prehormone that is produced in the liver. The mechanism of production includes hydroxylation of vitamin D3 (cholecalciferol) by the enzyme cholecalciferol 25-hydroxylase. Because it is the major form of vitamin D in the blood, it is considered the best indicator of vitamin D status. Calcidiol is converted in the kidneys into calcitriol which is the active form of vitamin D. 

Schools are being forced to embrace computers. unlike their "digital immigrant" teachers, these students are "digital natives", born into a world where technology is an integral part of everyday life. almost every student has access to a multitude of computers, laptops and tablet devices. it would be crazy to assume an assignment would  notbe written on computer in this day and age.many schools now have a 1-to-1 computer policy, and many districts and states in the united states have mandated computers or ipads across the state for every student,  including los angeles, the second largest school district in the country.but the thing is, when they tried it in la, it wasn't the success they hoped for. that's because the tablets weren't used as much as they expected. rather, it appeared there was a gap between the digital native's love of technology and their use of it in the classroom.this conflicted relationship with computers is also underscored by much of the literature on education. it is also highlighted in professional development and conferences such as  edutech.