Phosphorus forms a number of oxoacids. Out of these oxoacids,phosphinic acid $(H_3PO_2)$ has strong reducing properties. Write its structure and also write a reaction showing its reducing behavior.

The number of $P-H$,$P-O-P$,$P-O-H$ and $P=O$ bonds in sodium dihydrogen pyrophosphate $(Na_2H_2P_2O_7)$ are respectively:

There are some deposits of nitrates and phosphates in earth's crust. Nitrates are more soluble in water. Nitrates are difficult to reduce under the laboratory conditions but microbes do it easily. Ammonia forms a large number of complexes with transition metal ions. Hybridization easily explains the ease of sigma donation capability of $NH_3$ and $PH_3$. Phosphine is a flammable gas and is prepared from white phosphorus.
$1.$ Among the following,the correct statement is:
$(A)$ Phosphates have no biological significance in humans
$(B)$ Between nitrates and phosphates,phosphates are less abundant in earth's crust
$(C)$ Between nitrates and phosphates,nitrates are less abundant in earth's crust
$(D)$ Oxidation of nitrates is possible in soil
$2.$ Among the following,the correct statement is:
$(A)$ Between $NH_3$ and $PH_3, NH_3$ is a better electron donor because the lone pair of electrons occupies a spherical '$s$' orbital and is less directional
$(B)$ Between $NH_3$ and $PH_3, PH_3$ is a better electron donor because the lone pair of electrons occupies an $sp^3$ orbital and is more directional
$(C)$ Between $NH_3$ and $PH_3, NH_3$ is a better electron donor because the lone pair of electrons occupies an $sp^3$ orbital and is more directional
$(D)$ Between $NH_3$ and $PH_3, PH_3$ is a better electron donor because the lone pair of electrons occupies a spherical '$s$' orbital and is less directional
$3.$ White phosphorus on reaction with $NaOH$ gives $PH_3$ as one of the products. This is a:
$(A)$ dimerization reaction
$(B)$ disproportionation reaction
$(C)$ condensation reaction
$(D)$ precipitation reaction
Give the answer for questions $1, 2$ and $3.$

Which of the following elements of group $15$ $(VA)$ does not show allotropy?

Which of the following oxides is least acidic?

Give the preparation of phosphine $(PH_{3})$. State the physical and chemical properties of phosphine $(PH_{3})$. State the uses of phosphine.