QUES: The theory of special relativity finds a convenient formulation in Minkowski spacetime, a mathematical structure that combines three dimensions of space with a single dimension of time. In this formalism, distances in space can be measured by how long light takes to travel that distance, e.g., a light-year is a measure of distance, and a meter is now defined in terms of how far light travels in a certain amount of time. Two events in Minkowski spacetime are separated by an invariant interval, which can be either space-like, light-like, or time-like. Events that have a time-like separation cannot be simultaneous in any frame of reference, there must be a temporal component (and possibly a spatial one) to their separation. Events that have a space-like separation will be simultaneous in some frame of reference, and there is no frame of reference in which they do not have a spatial separation. Different observers may calculate different distances and different time intervals between two events, but the invariant interval between the events is independent of the observer (and his velocity).

What will events that have a time-like separation always be in some frame of reference?
What is the answer?
ANS: unanswerable
QUES: Epigenetic changes in eukaryotic biology serve to regulate the process of cellular differentiation. During morphogenesis, totipotent stem cells become the various pluripotent cell lines of the embryo, which in turn become fully differentiated cells. A single fertilized egg cell, the zygote, gives rise to the many different plant cell types including parenchyma, xylem vessel elements, phloem sieve tubes, guard cells of the epidermis, etc. as it continues to divide. The process results from the epigenetic activation of some genes and inhibition of others.

How are the different cells formed?
What is the answer?
ANS: continues to divide
QUES: Beginning with the Paris Exposition of 1878, the instrument's popularity rebounded. The Exposition was one of many stops for a popular new performing group the Estudiantes Espanoles (Spanish Students). They danced and played guitars, violins and the bandurria, which became confused with the mandolin. Along with the energy and awareness created by the day's hit sensation, a wave of Italian mandolinists travelled Europe in the 1880s and 1890s and in the United States by the mid-1880s, playing and teaching their instrument. The instrument's popularity continued to increase during the 1890s and mandolin popularity was at its height in "early years of the 20th century." Thousands were taking up the instrument as a pastime, and it became an instrument of society, taken up by young men and women. Mandolin orchestras were formed worldwide, incorporating not only the mandolin family of instruments, but also guitars, double basses and zithers.

When was it said that mandolin's popularity peaked?
What is the answer?
ANS:
early years of the 20th century.