What challenges cell biologists face in the 21st century?

There are many types of challenges are faced by the scientists in 21st century. some are as given below:

Explanation:

1. Getting biodiversity. We're losing species at a disturbing rate, and we have to comprehend what the implications of that may be.  

2. sustainable development. Like feasible agriculture, yet more broad sense. Discovering approaches to accommodate human needs while limiting the mischief we cause to the earth is urgent.  

3. Everything about hereditary qualities. It's simply so much fun.  

4. The most effective method to cut across political, social, and social limits and spur enormous and little scope activity to lessen biodiversity loss. This point may have a hefty sociology component, yet it's significant. All the natural and biological information on the planet won't make any difference on the off chance that we can't get individuals to follow up on it.

the answer is a or when it says crossing over

littering streams           is the answer