Neap tides, relatively weak tides, occur when the moon is in position(s)

a) a.
b) b.
c) a and c.
d) b and d.

D) B and D.  Weak tides, called neap tides, are experienced in the Earth's oceans when the sun is in positions B or D. In this case, the sun and moon interfere with each other in producing tidal bulges on Earth.  It is definitely not the other answer since I got it right on USATESTPREP.

answer : third trial had the contaminated sample and the correct value for dhrxn should be 1000 kj/mol

explanation :

let us write down the given equations

equation 1 :

equation 2 :

equation 3 :

let us use hess's law to find out which 2 equations can be added to get the third equation.

let us reverse equation 2 and multiply it by 2. the new equation that we get is,

equation 4 :

let us add equation 1 & 4

the above equation is similar to equation 3 but has different value for dhrxn.

therefore third trial had the contaminated sample and the correct value for dhrxn should be 1000 kj/mol