Consider the following species when answering the following question:
(i) nf3
(ii) bf3
(iii) if3
(iv) pbr3
(v) brcl3
of the molecules below, only is nonpolar.
bf3
nf3
pbr3
brcl3
if3

Only BF₃ is non polar.

Explanation:

The non polar molecule is one which will have zero dipole moment.

This zero dipole moment is possble if

a) the bonds are non polar (completely) and there is no lone electrons

b) the bonds are polar but the overall dipole moment cancel each other due to planar geometry of molecule

i) BF₃ : This is trigonal planar molecule. The overall dipole moment is zero due to symmetric distribution of electrons and thus dipole cancel each other.

ii) NF₃ : there is lone pair of electron and the bonds are polar.

iii) PBr₃: Again presence of lone pair of electrons and thus ssymmetric distribution of electrons

iv) BrCl₃ : The bonds are polar and molecule is not planar.

v) IF₃ : bonds are polar.

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1. the question asks the most appropriate, which seems to rule out "all of the above." your choice appears to be the correct one.

2. the protocol described is good for identifying correlation, but not cause and effect. for cause/effect, you need to devise a protocol that will assign the treatment (or not) to groups that have essentially the same "demographics" with respect to all the other variables. your choice regarding control group seems appropriate. random assignment of treatment (choice d) may be appropriate also. (i think you can argue that b and d are just different descriptions of the same method.)

3. d will randomize the birds, but may not guarantee there are 10 in each group. e will do both, so seems the better choice.

comment on question 2

based on the problem statement, it seems clear that confident, attentive drivers are more likely to use daytime running lights. so, the protocol might need to be devised in such a way that the lights would be turned on or off without the driver's knowledge. (an attentive driver will figure it out.) assigning a confident, attentive driver to a group required to drive without running lights might mess with his/her confidence and attentiveness. you don't want your experiment to do that. (nor do you want the experiment to subject a driver to greater risk of accident as a consequence of driving without running lights.) experiments involving human safety are always problematic.