Text B 　　 Among many scientists there is a concern about the genetic effects on the humanpopulation of increased exposure to radiation. This could come about through medical practiceor through the use of unclear power plants or weapons. The natural mutation rate can bedoubled by the low dose of 25 roentgens per generation. This increased mutation rate wouldgreatly increase the prenatal and postnatal death rates of human infants, as well as lead to morecases of deaf injuries, low-grade mental deficiency, and muscular symptoms, to name just threetypes of mutation for certain defects.
27. What is the main theme of this passage？
(A) The writer wants to alleviate worries about genetic mutation.
(B) The writer wants to show the invisible danger and risks of radiation at work places.
(C) The writer wants to identify the genetic mutations caused by radiation.
(D) The writer wants to convince people of the danger of genetic defects caused by radiation.