The reason why nost scientists thought that the first true cells formed on Earth were anaerobic is being shown in the second option :B. because there was no oxygen on early Earth. Paying attention to this fact scientists discovered that first cells were prokaryotes and greatest part of them created oxygen as a toxic by-product of their own metabolism which make them understand how the planet was provided with oxygen and made a conclusion that first cells were anaerobic.
Answer:
Covalent bonds form when two atoms share valence electrons,
while Van der Waals forces cause an attraction between nonpolar
molecules
Explanation:
A covalent bond is said to be formed when two atoms of the same or different elements share electrons. In an ordinary covalent bond, each bonding atom contributes one valence electron while in a dative covalent bond only one of the bonding atoms provides the shared pair of electrons.
Van Der Waals forces is the major intermolecular interaction between non polar compounds. Van Der Waals forces " are caused by correlations in the fluctuating polarizations of nearby particles"(Wikipedia).
A carbon atom has 4 valence electrons
Physician recommends radiation therapy because radiation therapy helps in preventing cellular growth. It uses high-energy radiation to shrink tumor or kill cancer cells. It may be used to cure or to control malignancy when the tumor can no longer be removed or when lymph node involvement is present; also, it can be used prophylactically to prevent spread. Radiation therapy kills cancer cells by damaging their DNA which is the molecules inside cells that carry genetic information and pass it from generation to generation. Radiation therapy can either damage DNA directly or create charged particles within the cells that can in turn damage the DNA. Cancer cells whose DNA is damaged beyond repair stop dividing or die and when these cells die, they are eliminated by the body through natural process.
