Given below is a flow chart showing ovarian changes during the menstrual cycle. Fill in the spaces by providing the names of the hormones responsible for the events shown.
$\boxed{\text{Primary follicles}}$
$\downarrow \leftarrow \boxed{A}$
$\boxed{\text{Graafian follicle}}$
$\downarrow \leftarrow \boxed{B}$
$\boxed{\text{Ovum}}$
$\downarrow \leftarrow \boxed{C}$
$\boxed{\text{Corpus luteum}}$

(A-D) The correct hormones are: $A = FSH$ and $LH$,$B = LH$,$C = LH$.
$1$. During the follicular phase,the secretion of gonadotropins ($FSH$ and $LH$) increases gradually. $FSH$ stimulates the growth and development of primary follicles into Graafian follicles.
$2$. Rapid secretion of $LH$ ($LH$ surge) during the mid-cycle (around the $14$th day) induces the rupture of the Graafian follicle,leading to ovulation (release of the ovum).
$3$. After ovulation,the remaining parts of the Graafian follicle transform into the corpus luteum under the influence of $LH$. The corpus luteum then secretes large amounts of progesterone,which is essential for the maintenance of the endometrium.