Why do you think Bernstein refers so often to Engels?

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.

When two vehicles meet on a steep road where neither can pass: the vehicles going downhill must back up or simply must yield the right of way so it can pass : )