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In `1894`, Lord Rayleigh reported that the mass of chemically prepared nitrogen was different from that of nitrogen extracted from the atmosphere, as shown in Tables `1` and `2`. Latter, this difference was attributed to the presence of argon in atmospheric nitrogen. The masses of gases were mearsured by using a glass vessel with a known volume under atmospheric pressure `(1.013xx10^(5)Pa)`.
`|{:("From nitric oxide",,2.3001 g),("From nitrous oxide",,2.2990 g),("From amonium nitrite purified at a red heat",,2.2987 g),("From urea",,2.2985 g),("From ammonium nitrite purified in the cold",,2.2987 g),("Mean",,2.2990 g):}|`
`|{:(O_(2) "was removed by hot copper" (1892),,2.3103 g),(O_(2) "was removed by hot iron" (1893),,2.3100 g),(O_(2) "was removed by ferrous hydrate (1894),2.3102 g,),(Mean,,2.3102 g):}|`
Ramsay and cleve discovered helium in cleveite (a mineral consiting of uranium oxide and oxides of lead, thorium, and rare earths, an impure variety of uraninite) independently and virtually simultaneously in `1895`. The gas extracted from the rock showed a unique spectroscopic line at around `588` nm (indicated by `D3` in Figure `1`), which was fist observed in the spectrum of solar prominence during a total eclipse in `1868`, near the well-known `D_(1)` and `D_(2)` lines of sodium.
image
`ul("Which")` equation explains the occurrence of helium in cleveite among `[A]` to `[D]` below? Mark one.
A. `.^(238)U rarr .^(234)Th+alpha`
B. `Uhe_(2) rarr U+2 He`
C. `.^(240)U rarr .^(240)Np+beta^(-)`
D. `.^(235)U+n rarr .^(95)Y+^(139)l+2 n`

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Correct Answer - Considering that the a particle is the nucleus of helium a decay `[A]` is the relevent source of helium in such rocks. No compounds of He such as `UHe2` in `[B]` is known to be stable at ambient temperature. `[C]` is a radioactive decay of `240 U` in the thorium series. `[D]` is a nuclear fission reaction of `235U` occurring in nuclear reactors. Thus, the correct answer is `[A]`

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