What does it mean for an organ to have the quality of "irreducible complexity"?

The cluster of developing cells from conception until birth is called an

One important development during the 3rd trimester that will insulate the infant against changes in temperature is a. the deposition of fat under the skin b. the closing of the septum between c. the atria of the heart d. the beginnings of lung functioning e. the beginnings of light and sound sensing

Which of the following particles always has a positive charge? a. electronb. neutron c. proton d. ion