Which of the following changes will always increase the efficiency of a thermodynamic engine? choose all correct statements.

check all that apply.
a. decrease the hot reservoir temperature th while keeping the cool reservoir temperature tc constant.
b. increase the difference th−tc.
c. decrease the ratio tc/th.
d. increase th while keeping tc constant. increase tc while keeping th constant. decrease tc while keeping th constant.

B,C,D

Explanation:

Thermodynamic  efficiency is given by

efficiency can be increased by Keeping constant and increasing

Keeping constant and decreasing

by increasing

by decreasing ratio          

a.metals can become magnetic if they have many spinning electrons that create magnetic fields.d.only some metals are attracted by magnets; these metals are called ferromagnetic.

d.wrapping an insulated wire around a metal with ferromagnetic properties and applying an electric current.