In a twin study, an investigator compares concordance rates of fraternal and identical twins. a concordance rate is best described as

The answer should be b.

Answer(a) one needs to learn from experts and be determined to be the best in the field;

So what is the question

Some connecting rods have to lubricate the cylinder wall or piston pin. the answer is letter d. graphite coating can be a major  additive to many  coating  systems and things such as connecting rods.  it functions as a refractory, lubricant, thermal conductor, and electrical conductor for a machine or thing to enhance its work.