Why can black holes not be seen?

they have too much gravity for light to escape.
they are hidden in interstellar dust.
they exist only behind other stars.
scientists don’t know where to look for them.

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

People with one sickle cell allele are not likely to get malaria. what is this an example of?

How do the sperm cells get from the stigma to the ovules? a. they slide down the petals to the bottom of the flower. b. they travel through pollen tubes. c. they travel along filaments. d. insects carry the sperm cells from the stigma to the ovules.