Questions

Tax that uses the same rate for all income levels is proportional tax.

When water is cooled from 4°C to 0°C, its density decreases.

When water cools from 4°C to 0°C, ice crystals begin to form because the water is approaching its freezing point. When water freezes, the water molecules form a lattice that increases the volume. As a result, the density decreases. This is why ice will float on top of water; ice has a lower density than the water.

A clade is a group of ancestral species and all its descendants. Hence it is a branch of one same species and so monophyletic.

The main source of organic matter in soil is the plant and animal tissue that decompose at each level of breakdown.

Kepler’s  laws  of  planetary  motion state  that  (a)  All  planets  move  in elliptical orbits with the Sun at one of  the  focal  points  (b)  The  radius vector  drawn  from  the  sun  to  a planet  sweeps  out  equal  areas  in equal  time  intervals.  This  follows from  the  fact  that  the  force  of gravitation on the planet is central and  hence  angular  momentum  is conserved.  (c)  The  square  of  the orbital   period   of   a   planet   is proportional  to  the  cube  of  the semi  major  axis  of  the  elliptical orbit of the plane.