Which one of the following is correct?

Cardiac activities of the heart are regulated by: A. Nodal tissue B. A special neural centre in the medulla oblongata C. Adrenal medullary hormones D. Adrenal cortical hormones Choose the correct answer from the options given below :

Match List I with List II : \begin{array}{|l|l|l|l|}\hline & List I & & List II \\\hline A. & P wave & I. & Heart muscles are electrically silent. \\\hline B. & QRS complex & II. & Depolarisation of ventricles. \\\hline C. & T wave & III. & Depolarisation of atria. \\\hline D. & T-P gap & IV. & Repolarisation of ventricles. \\\hline\end{array} Choose the correct answer from the options given below :

The mother has $\mathrm{A}+$ blood group the father has $\mathrm{B}+$ and the child is $\mathrm{A}+$. What can be the possibility of genotypes of all three, respectively? A. $\quad I^A I^A\left|I^B i\right| I^B i$ B. $\quad I^A I^A\left|I^B i\right| I^A i$ C. $\quad I^B i\left|I^A I^A\right| I^A I^B$ D. $\quad I^A I^A\left|I^B I^B\right| I^A i$ E. $\quad I^A i\left|I^B i\right| I^{A_i}$ Choose the correct answer from the option given below:

'Lub' sound of Heart is caused by the ______

Given below are two statements : Statement I : Bone marrow is the main lymphoid organ where all blood cells including lymphocytes are produced. Statement II : Both bone marrow and thymus provide micro environments for the development and maturation of T-lymphocytes. In the light of above statements, choose the most appropriate answer from the options given below :